Fix flaky spec in user_posts_notes_spec.rb

The test was previously failing because it was possible for a note to be
deleted before the browser rendered the page, so there was note to
reply.  We fix this by deleting the note only after the page has been
displayed and the user has clicked the reply button.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/238463

spec/features/merge_request/user_posts_notes_spec.rb

@@ -108,15 +108,14 @@ RSpec.describe 'Merge request > User posts notes', :js do
     end
 
     context 'when comment is deleted' do
-      before do
-        note.delete
-      end
-
       it 'shows an error message' do
         find('.js-reply-button').click
 
         page.within('.discussion-reply-holder') do
           fill_in 'note[note]', with: 'A reply'
+
+          note.delete
+
           click_button 'Add comment now'
 
           expect(page).to have_content('Your comment could not be submitted because discussion to reply to cannot be found')