Remove redundancy in /roompromote

If used to promote a user to room owner, the command displayed: "User was promoted to Room Room Owner". There were also two error messages for trying to promote to global-only ranks, so I kept the newer one from <a href="https://github.com/Zarel/Pokemon-Showdown/commit/4f86a6e849a731f0e3359d1c135ca6d40594702a">this commit.</a>