OneOf is set multiple times with different values

These calls to OneOf setters in `BigtableTableAdminClientWrapper.java` overwrite previously set values:

https://github.com/googleapis/java-bigtable-hbase/blob/d3403e14cf0463d39e71d1af62a81f3c4110d125/bigtable-client-core-parent/bigtable-client-core/src/main/java/com/google/cloud/bigtable/grpc/BigtableTableAdminClientWrapper.java#L240

https://github.com/googleapis/java-bigtable-hbase/blob/d3403e14cf0463d39e71d1af62a81f3c4110d125/bigtable-client-core-parent/bigtable-client-core/src/main/java/com/google/cloud/bigtable/grpc/BigtableTableAdminClientWrapper.java#L258

This was noticed because of an [Error Prone static check](https://errorprone.info/bugpattern/RedundantSetterCall), which reports the following error for that code:

```
src/main/java/com/google/cloud/bigtable/grpc/BigtableTableAdminClientWrapper.java:240: error: [RedundantSetterCall] The oneof `Target` (set via setDeleteAllDataFromTable, setRowKeyPrefix) was called twice with . Setting the same field multiple times is redundant, and could mask a bug.
.setRowKeyPrefix(rowKeyPrefix)
^
(see https://errorprone.info/bugpattern/RedundantSetterCall)
src/main/java/com/google/cloud/bigtable/grpc/BigtableTableAdminClientWrapper.java:258: error: [RedundantSetterCall] The oneof `Target` (set via setDeleteAllDataFromTable, setRowKeyPrefix) was called twice with . Setting the same field multiple times is redundant, and could mask a bug.
.setRowKeyPrefix(rowKeyPrefix)
^
(see https://errorprone.info/bugpattern/RedundantSetterCall)
```