Allow removing a step from workspace's cache

### 🚀 The feature, motivation and pitch

Goal: API for removing a step from a workspace's cache.

1. Add `workspace.remove_step()`
2. Implement method for all local and remote workspaces.
3. Expose API to CLI: eg. `tango workspace remove ` (will be useful to expose other workspace functions like `list runs` and `list steps` also).

### Alternatives

Manually delete from workspace. This can be done for local workspace, since it only requires deleting folders, but not very easily for remote workspaces, which may store step info in datastores and the actual data in cloud buckets.

### Additional context

We have run into situations where this would be useful : https://github.com/allenai/tango/issues/413