Reconsider TabletEnvironmentIterator constructor

[dlmarion]

In #5490 a constructor was added to TabletIteratorEnvironment to support its use in tests. The thought was to use server-side IteratorEnvironment implementations in tests where possible instead of using the new ClientIteratorEnvironment.

#5490 (comment)

Consider removing the constructor, or adding a VisibleForTesting annotation.

[dlmarion]

Closed by #5587