Remove feature flag for limiting diverging commit counts

In GitLab 12.4 via !16737 (merged), we required the frontend to request at most 20 branches in the Projects::BranchesController#diverging_commit_counts endpoint to avoid Gitaly N+1 issues. This commit removes the feature flag and makes this behavior mandatory. In the past week, there do not appear to be any rejections, which would show up as 422 errors. (https://log.gitlab.net/goto/a73fd212fd747a16bbf3e9559a4c56f6)