Revert "Merge branch '28682-can-merge-branch-before-build-is-started' into 'master'"

This reverts commit 793be43b, reversing
changes made to 8d0b4872.

For projects not using any CI, enabling merge only when pipeline succeeds
caused merge requests to be in unmergeable state, which caused significant
confusion.

See https://gitlab.com/gitlab-org/gitlab-ce/issues/55144 for more details.

app/models/merge_request.rb

@@ -978,6 +978,7 @@ class MergeRequest < ActiveRecord::Base
 
   def mergeable_ci_state?
     return true unless project.only_allow_merge_if_pipeline_succeeds?
+    return true unless head_pipeline
 
     actual_head_pipeline&.success? || actual_head_pipeline&.skipped?
   end

changelogs/unreleased/28682-can-merge-branch-before-build-is-started.yml

----
-title: Strictly require a pipeline to merge.
-merge_request: 22911
-author:
-type: changed

spec/models/merge_request_spec.rb

@@ -1885,7 +1885,7 @@ describe MergeRequest do
           allow(subject).to receive(:head_pipeline) { nil }
         end
 
-        it { expect(subject.mergeable_ci_state?).to be_falsey }
+        it { expect(subject.mergeable_ci_state?).to be_truthy }
       end
     end