The refactoring in 0a8db3dc
inadvertently caused commit checks to become active regardless of
whether any rules were actually active. This significantly slowed pushes
and caused repositories with large number of commits to time out.

We fix this by using `push_rule_checks_commit?` since the existence of a
`PushRule` is not sufficient to determine whether any rules are
active. This also eliminates the need to override
`should_run_diff_validations?`, since the set of validations will only
contain the active ones.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/57067