Remove memoizing of source_branch_exists from MergeRequest

We are now memozing at a higher level, so we can remove it from
MergeRequest.

Merge Request: gitlab.com/gitlab-org/gitlab/-/merge_requests/39944

app/models/merge_request.rb

@@ -105,7 +105,6 @@ class MergeRequest < ApplicationRecord
 
   after_create :ensure_merge_request_diff
   after_update :clear_memoized_shas
-  after_update :clear_memoized_source_branch_exists
   after_update :reload_diff_if_branch_changed
   after_commit :ensure_metrics, on: [:create, :update], unless: :importing?
   after_commit :expire_etag_cache, unless: :importing?
@@ -867,10 +866,6 @@ class MergeRequest < ApplicationRecord
     clear_memoization(:target_branch_head)
   end
 
-  def clear_memoized_source_branch_exists
-    clear_memoization(:source_branch_exists)
-  end
-
   def reload_diff_if_branch_changed
     if (saved_change_to_source_branch? || saved_change_to_target_branch?) &&
         (source_branch_head && target_branch_head)
@@ -1122,12 +1117,10 @@ class MergeRequest < ApplicationRecord
   end
 
   def source_branch_exists?
-    strong_memoize(:source_branch_exists) do
-      next false unless self.source_project
+    return false unless self.source_project
 
     self.source_project.repository.branch_exists?(self.source_branch)
   end
-  end
 
   def target_branch_exists?
     return false unless self.target_project

spec/models/merge_request_spec.rb

@@ -1257,10 +1257,8 @@ RSpec.describe MergeRequest do
     let(:repository) { merge_request.source_project.repository }
 
     context 'when the source project is set' do
-      it 'memoizes the value and returns the result' do
-        expect(repository).to receive(:branch_exists?).once.with(merge_request.source_branch).and_return(true)
-
-        2.times { expect(merge_request.source_branch_exists?).to eq(true) }
+      it 'returns true when the branch exists' do
+        expect(merge_request.source_branch_exists?).to eq(true)
       end
     end
 

spec/services/merge_requests/conflicts/list_service_spec.rb

@@ -30,7 +30,6 @@ RSpec.describe MergeRequests::Conflicts::ListService do
     it 'returns a falsey value when one of the MR branches is missing' do
       merge_request = create_merge_request('conflict-resolvable')
       merge_request.project.repository.rm_branch(merge_request.author, 'conflict-resolvable')
-      merge_request.clear_memoized_source_branch_exists
 
       expect(conflicts_service(merge_request).can_be_resolved_in_ui?).to be_falsey
     end