Remove GET /roles/{role_id}/users (bug 1183433)

This resources was unimplemented in grizzly, and the semantics of the
call need to be carefully considered (and perhaps redesigned) prior to
implementation.

Change-Id: I5c26a5d248c774143dae0e648090e209bc94899e
Merge "Remove GET /roles/{role_id}/users (bug 1183433)"