Remove ci_pipelines -> external_pull_requests LFK

We defined external_pull_requests as a CI table, and so we kept the FK.
So there's no need for this LFK

config/gitlab_loose_foreign_keys.yml

@@ -78,9 +78,6 @@ ci_pipelines:
   - table: merge_requests
     column: merge_request_id
     on_delete: async_delete
-  - table: external_pull_requests
-    column: external_pull_request_id
-    on_delete: async_nullify
   - table: users
     column: user_id
     on_delete: async_nullify

spec/models/external_pull_request_spec.rb

@@ -233,10 +233,6 @@ RSpec.describe ExternalPullRequest do
     end
   end
 
-  it_behaves_like 'it has loose foreign keys' do
-    let(:factory_name) { :external_pull_request }
-  end
-
   context 'loose foreign key on external_pull_requests.project_id' do
     it_behaves_like 'cleanup by a loose foreign key' do
       let!(:parent) { create(:project) }