Replace `wait_for_texts_not_to_be_visible` with a check for branches in `qa/qa/specs/features/browser_ui/3_create/repository/add_list_delete_branches_spec.rb`

In gitlab-ce!23850 Page::Project::Branches::Show#wait_for_texts_not_to_be_visible was added in response to a discussion. The method is used to check that branches have been deleted. It does so by checking for the absence of commit messages for the latest commit to each branch.

It might be better to check for the branches themselves (e.g., via the link text in the branch-title elements) because the commit messages might not be unique.