Remove feature flag for limiting diverging commit counts

In GitLab 12.4 via
https://gitlab.com/gitlab-org/gitlab/merge_requests/16737, 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.

app/controllers/projects/branches_controller.rb

@@ -133,8 +133,6 @@ class Projects::BranchesController < Projects::ApplicationController
   # frontend could omit this set. To prevent excessive I/O, we require
   # that a list of names be specified.
   def limit_diverging_commit_counts!
-    return unless Feature.enabled?(:limit_diverging_commit_counts, default_enabled: true)
-
     limit = Kaminari.config.default_per_page
 
     # If we don't have many branches in the repository, then go ahead.

changelogs/unreleased/sh-remove-feature-flag-diverging-commits.yml

+---
+title: Remove feature flag for limiting diverging commit counts
+merge_request: 20999
+author:
+type: other