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)
Edited by Robert Speicher