git: Remove feature flag for bundled Git

Since 073727ab (git: Add a feature flag to toggle between bundled and external Git, 2022-01-13) we have now carried a feature flag that allowed us to ease into the deployment of bundled Git in production. With this new Git execution environment, Gitaly doesn't need a full Git distribution anymore but can instead have multiple sets of bundled Git binaries installed into its own binary directory from which it can bootstrap a complete Git environment. This allows us to use feature flagged rollouts of new Git versions in production and fixes issues with zero-downtime upgrades.

Bundled Git has been rolled out for a full month by now and is default enabled since 52625870 (git: Enable use of bundled Git by default, 2022-02-28). Let's consider it stable and remove the feature flag altogether. In the worst case, users can still disable bundled Git via the Gitaly configuration anyway.

Changelog: changed

Closes #4064 (closed)