Cleanup unused css and helper methods due to issues bulk edit cleanup

We removed bulk edit related haml templates in scope of pajamas migration here !146810 (merged)

This is a follow-up ticket to cleanup all unused css and helper methods due to above change:

  1. Remove unused css classes

Suggestion (non-blocking): I did a search for dropdown-new-label and there seems to be a bit of CSS / JS that is related and can potentially be removed. I think that might be a bit too much to add to the MR though so don't mind if we create a follow-up to look into it in a separate MR. If we create an issue, we should go through all the removed classes since I see some like default-dropdown-input and dropdown-label-color-input that can also be removed.

  1. Remove unused helper methods

Nit: I believe we no longer use this method manage_labels_title so it can be removed as well.