Remove gitaly_ruby_remote_branches_ls_remote flag

This behavior has been battle-tested on gitlab.com for almost two months, and was included in the 13.2 on-premises release defaulted to on.

Pending any last-minute catastrophic failures, we can safely remove the old fetch-based behavior.

This is the final step for gitaly#2670 (closed). 🎉

Gitaly counterpart at gitaly!2417 (merged)