Remove unnecessary existence check on persistent ref

This commit reduces the number of gitaly calls

app/models/ci/persistent_ref.rb

@@ -22,7 +22,7 @@ module Ci
     end
 
     def create
-      return unless enabled? && !exist?
+      return unless enabled?
 
       create_ref(sha, path)
     rescue => e

spec/models/ci/persistent_ref_spec.rb

@@ -73,8 +73,8 @@ describe Ci::PersistentRef do
         pipeline.persistent_ref.create
       end
 
-      it 'does not create a persistent ref' do
-        expect(project.repository).not_to receive(:create_ref)
+      it 'overwrites a persistent ref' do
+        expect(project.repository).to receive(:create_ref).and_call_original
 
         subject
       end