Investigate dropping gpgme dependencies from build

In #2836 (comment 42516970), we added gpgme libraries via !2109 (merged).

However, ruby-gpgme already will download and bundle its dependencies (https://github.com/ueno/ruby-gpgme/issues/154) if the --use-system-libraries config is NOT added.

In fact, it may be preferable for ruby-gpgme to do this since it knows which versions that it is tested and compatible with.

The gpgme libraries do install GPG binaries that GitLab Rails might need, but I would hope that the ruby-gpgme just uses the gpgme libraries as an interface.