Remove mistaken support for `gitlab-shell user-123`

In !226 (merged) we added tests that assert that bin/gitlab-shell can be invoked with user-123 as an argument. This is bogus. The bin/gitlab-shell executable is only invoked by sshd, via the authorized_keys file, AuthorizedKeysCommand or AuthorizedPrincipalsCommand. None of these invocations "know" the gitlab user ID.

Note that user-123 is a valid GL_ID identifier. However, these are only use by Git hooks (in Gitaly). The GL_ID identifier is not known at the time an SSH session is established, which is when bin/gitlab-shell runs.

Closes #151 (closed)