Migrate most of XWiki's LESS files to native CSS

For action-menus.css , there's a huge block with complex functions and mixins.
Those rules are applied on `.navbar-nav > .dropdown-split` . `.dropdown-split` is found in only one place currently in XWiki, the velocimacro `xwikitopmenuentrystart` defined in menus_macro.vm.
The velocimacro is not used in XWiki.
In order to understand what it was used for, I logged commits with `git log -S "xwikitopmenuentrystart"`. The latest commit that removed uses of the velocimacro was:
https://github.com/xwiki/xwiki-platform/commit/8820a51f8a0bbf22b23549d0c54475ae2e28c9c8#diff-e72d71f4fe47cdd456fbc3054759f99b5a608ea35783ca52a3fd06e58b5cf304L19

AFAIU, this velocimacro was used in Colibri and never in Flamingo. Colibri was retired in 2015.
The velocimacro and styles are very specific / complex and I doubt a lot of customization would rely on it.

IMO it's alright to consider this velocimacro and styles are not API and remove them.
In regards to the discussion at https://forum.xwiki.org/t/state-of-support-of-the-colibri-skin/16494 where we established that it was okay to remove code specific to Colibri, I'll take the liberty to remove the code for the velocimacro and the styles that go along with it while migrating.