Stop building non-embedded Git binaries

Context

When Gitaly is packaged for customer deployments, for example into Linux packages via Omnibus, the versions of Git that Gitaly depends on are compiled separately.

With !7035 (merged) and #6089 (closed), we've started to embed Git binaries inside the Gitaly binary. When #6194 (closed) is complete, we'll no longer need to ship separate Git binaries to customers.

One concern is that removing these binaries will cause issues for customers who don't have use_bundled_binaries enabled in their Gitaly config.toml. However, this was set as the default a while ago: gitlab-org/charts/gitlab#3029 (closed)

Proposal

Scour the various Gitaly packaging mechanisms and stop building separate Git binaries.