Revert "Merge branch 'sh-set-alternate-refs-prefixes-gitconfig' into 'master'"

This reverts merge request !3364 (merged)

The merge request that introduced this change was more of an hotfix, but long term it's better to remove the config value as Gitaly now manages the config value again, and does so in a better way. Also, in the future it's probably a good idea to actually expose the refs from the pool repository's origin remote, as that would speed up clients pushes.

Gitaly MR: gitaly!1323 (merged)