Drop feature checks from WorkspaceImpl

Many of the WorkspaceImpl methods use the ensureSupportedOption() method to check whether a particular repository descriptor is present. That's a bit backwards, as the presence of those descriptors should be based on the functionality available through the Workspace implementation, not the other way around.

Such checks make sense in cases like remoting layers where the features of the underlying implementation are unknown, but in a case like ours where we do know the underlying implementation so instead of inspecting the descriptors the WorkspaceImpl methods can either proceed directly to the implementation or just throw an UnsupportedRepositoryOperationException.