Mailroom: Remove the wrapper script

• This fixes an issue where the ruby process would start, but if it were to fail, we lose that context, and container would continue to run despite the intent of this container no longer doing any work
• I'm not sure why the wrapper script or the creation of a log file are necessary
• mail_room outputs to stdout by default
  • Our helm charts also do not configure the log_path option
  • due to this I elected to start the ruby process natively
  • output goes to stdout and if the process dies for any reason, so will the container

Addresses: gitlab-org/charts/gitlab#1670 (closed)