Fix failure in spec/features/unsubscribe_links_spec.rb

Not sure why this fails with the full login flow, but it's not necessary
and would've been changed eventually anyway, so whatever!

spec/features/unsubscribe_links_spec.rb

@@ -57,7 +57,7 @@ describe 'Unsubscribe links', feature: true do
 
   context 'when logged in' do
     before do
-      gitlab_sign_in(recipient)
+      sign_in(recipient)
     end
 
     it 'unsubscribes from the issue when visiting the link from the email body' do