Remove allow_n_plus_1 block from DeleteMergedBranchesService

Fixes #37438
Fixes gitaly#999

app/services/delete_merged_branches_service.rb

@@ -6,10 +6,7 @@ class DeleteMergedBranchesService < BaseService
   def execute
     raise Gitlab::Access::AccessDeniedError unless can?(current_user, :push_code, project)
 
-    # n+1: https://gitlab.com/gitlab-org/gitlab-ce/issues/37438
-    Gitlab::GitalyClient.allow_n_plus_1_calls do
-      branches = project.repository.branch_names
-      branches = branches.select { |branch| project.repository.merged_to_root_ref?(branch) }
+    branches = project.repository.merged_branch_names
     # Prevent deletion of branches relevant to open merge requests
     branches -= merge_request_branch_names
     # Prevent deletion of protected branches
@@ -19,7 +16,6 @@ class DeleteMergedBranchesService < BaseService
       DeleteBranchService.new(project, current_user).execute(branch)
     end
   end
-  end
 
   private