objectpool: Enable rev-list based connectivity checks

When disconnecting a repository from its object pool we first copy over all objects and then remove the gitalternates file. In order to verify that there wasn't any race during the update we will perform a connectivity check after the gitalternates file was removed, and if it fails, we move the gitalternates file back into place.

This connectivity check was implemented via git-fsck(1). But after some upstream discussion in the Git project 1 it was noted that doing connectivity checks is both faster and less memory intensive when using git-rev-list(1) instead.

Quite a while ago, in v15.5.0, we have thus introduced the new rev-list based connectivity check, but didn't roll the flag out due to whatever reason. We have now done so almost a week ago and didn't observe any issues caused by the new connectivity check. So let's remove the feature flag and unconditionally enable the new, more performant connectiviyt check.

Closes #4489 (closed).