repository: Remove feature flag for atomic repo creation from bundles

When not using the --atomic flag for git-fetch(1), then Git will create a separate reference transaction per reference that is to be fetched. This can be extremely expensive in case we fetch thousands or even tens of thousands of references given that we'll have to execute the gitaly-hooks binary each time, connect to the hooks service, cast votes and then wait for quorum. This was fixed via the introduction of CreateRepositoryFromBundleAtomicFetch, which causes us to pass that flag.

This feature has been default-enabled for three weeks in production now, and furthermore it's been backported to v13.12 and v14.0. No problems were observed in production, and it fixed the performance issue for customers.

Given its exposure, let's remove the feature flag without taking the intermediate step of first default-enabling it.

Changelog: performance

Closes #3686 (closed)