Call after_remove_branch only once after importing all GitHub PRs

CHANGELOG

@@ -7,6 +7,7 @@ v 8.13.0 (unreleased)
   - Fix robots.txt disallowing access to groups starting with "s" (Matt Harrison)
   - Revoke button in Applications Settings underlines on hover.
   - Add organization field to user profile
+  - Optimize GitHub importing for speed and memory
 v 8.12.2 (unreleased)
   - Fix Import/Export not recognising correctly the imported services.

lib/gitlab/github_import/importer.rb

@@ -108,6 +108,8 @@ module Gitlab
             end
           end
         end
+
+        project.repository.after_remove_branch
       end
       def restore_source_branch(pull_request)
@@ -127,8 +129,6 @@ module Gitlab
       def clean_up_restored_branches(pull_request)
         remove_branch(pull_request.source_branch_name) unless pull_request.source_branch_exists?
         remove_branch(pull_request.target_branch_name) unless pull_request.target_branch_exists?
-
-        project.repository.after_remove_branch
       end
       def apply_labels(issuable, raw_issuable)