Remove v3/projects/{project_id}/users from Identity v3

There was documentation for `GET v3/projects/{project_id}/users`,
but there's no v3/projects/{project_id}/users resource in the
Identity API V3. When users try the operation they just get a
404 Not Found response.

This change removes the `GET v3/projects/{project_id}/users` from
the api reference.

Change-Id: I51d8b36e657d2a8decc8ffd5e68f021f897ab209
Closes-Bug: #1307699
Merge "Remove v3/projects/{project_id}/users from Identity v3"