Add remove-clusters command for namespace in pulsar-admin

**Is your feature request related to a problem? Please describe.**
CLI `bin/pulsar-admin` supports `set-clusters` and `get-clusters` command so that we can `set` / `get` replication clusters for a namespace. But it lacks corresponding `remove-clusters` command to restore to the unset state, I think it is necessary to add this command to ensure the closed-loop operation of the replication cluster.

**Describe the solution you'd like**
### In Client Side
Add a inner class `RemoveReplicationClusters` in [CmdNamespaces.java](https://github.com/apache/pulsar/blob/master/pulsar-client-tools/src/main/java/org/apache/pulsar/admin/cli/CmdNamespaces.java) as below:
```
private class RemoveReplicationClusters extends CliCommand {
@Parameter(description = "tenant/namespace", required = true)
private java.util.List params;

@Override
void run() throws PulsarAdminException {
String namespace = validateNamespace(params);
getAdmin().namespaces().removeNamespaceReplicationClusters(namespace);
}
}
```
Then add two methods `removeNamespaceReplicationClusters` and `removeNamespaceReplicationClustersAsync` in [Namespaces.java]( https://github.com/apache/pulsar/blob/master/pulsar-client-admin-api/src/main/java/org/apache/pulsar/client/admin/Namespaces.java) as below:
```
/**
* Remove the replication clusters for a namespace.
*
* @param namespace
* @throws PulsarAdminException
*/
void removeNamespaceReplicationClusters(String namespace) throws PulsarAdminException;

/**
* Remove the replication clusters for a namespace asynchronously.
*
* @param namespace
* @return
*/
CompletableFuture removeNamespaceReplicationClustersAsync(String namespace);
```
and implement them by calling an asynchronous delete request in [NamespacesImpl.java](https://github.com/apache/pulsar/blob/master/pulsar-client-admin/src/main/java/org/apache/pulsar/client/admin/internal/NamespacesImpl.java) as below:
```
@Override
public void removeNamespaceReplicationClusters(String namespace) throws PulsarAdminException {
sync(() -> removeNamespaceReplicationClustersAsync(namespace));
}

@Override
public CompletableFuture removeNamespaceReplicationClustersAsync(String namespace) {
NamespaceName ns = NamespaceName.get(namespace);
WebTarget path = namespacePath(ns, "replication");
return asyncDeleteRequest(path);
}
```
### In Server Side
Add method `removeNamespaceReplicationClusters` in [v1/Namespaces.java](https://github.com/apache/pulsar/blob/master/pulsar-broker/src/main/java/org/apache/pulsar/broker/admin/v1/Namespaces.java) and [v2/Namespaces.java](https://github.com/apache/pulsar/blob/master/pulsar-broker/src/main/java/org/apache/pulsar/broker/admin/v2/Namespaces.java) as below:
```
// for v1
@DELETE
@Path("/{property}/{cluster}/{namespace}/replication")
@ApiOperation(value = "Remove the replication clusters for namespace")
@ApiResponses(value = {@ApiResponse(code = 403, message = "Don't have admin permission"),
@ApiResponse(code = 404, message = "Tenant or cluster or namespace doesn't exist"),
@ApiResponse(code = 412, message = "Namespace is not global")})
public void removeNamespaceReplicationClusters(@PathParam("property") String property,
@PathParam("cluster") String cluster,
@PathParam("namespace") String namespace) {
validateNamespaceName(property, cluster, namespace);
internalSetNamespaceReplicationClusters(Lists.newArrayList());
}

// for v2
@DELETE
@Path("/{tenant}/{namespace}/replication")
@ApiOperation(value = "Remove the replication clusters for namespace")
@ApiResponses(value = {@ApiResponse(code = 403, message = "Don't have admin permission"),
@ApiResponse(code = 404, message = "Tenant or cluster or namespace doesn't exist"),
@ApiResponse(code = 412, message = "Namespace is not global")})
public void removeNamespaceReplicationClusters(@PathParam("tenant") String tenant,
@PathParam("namespace") String namespace) {
validateNamespaceName(tenant, namespace);
internalSetNamespaceReplicationClusters(Lists.newArrayList());
}
```
and they can reuse the `internalSetNamespaceReplicationClusters` method directly in [NamespacesBase.java](https://github.com/apache/pulsar/blob/master/pulsar-broker/src/main/java/org/apache/pulsar/broker/admin/impl/NamespacesBase.java), just pass a `empty List`. The **important thing** is `replicationClusterSet` in method `internalSetNamespaceReplicationClusters` should be consistent with the initial value When creating replication clusters, its value is `empty Set` in v1/namespace as below:
https://github.com/apache/pulsar/blob/9994614173205abd075fcc670396cebd71227047/pulsar-client-admin-api/src/main/java/org/apache/pulsar/common/policies/data/Policies.java#L38
and its value is `local cluster` in v2/namespace as below:
https://github.com/apache/pulsar/blob/9994614173205abd075fcc670396cebd71227047/pulsar-broker/src/main/java/org/apache/pulsar/broker/admin/impl/NamespacesBase.java#L2025-L2029
So we modify the value of `replicationClusterSet` in method `internalSetNamespaceReplicationClusters` but this will not affect this method as below:
```
Set replicationClusterSet = clusterIds.isEmpty()
? namespaceName.isV2()
? Sets.newHashSet(config().getClusterName()) : Sets.newHashSet()
: Sets.newHashSet(clusterIds);
```