Disable RubyGems for Gitaly gitlab-shell hooks

By default, RubyGems is enabled, which causes the Ruby interpreter to load 600+ additional files at startup, even though gitlab-shell is designed not to use any external gems.

This was done in !1707 (diffs) for gitlab-shell, but when the hooks migrated to Gitaly we lost the performance optimization.

Also remove gitlab-shell wrapper since it's no longer needed; the hooks have been replaced with shell scripts;

Closes gitaly#2660 (closed)