Drop support for LESS in XWiki

This is the last step of a multiple step process. This was not yet voted, and should be discussed again once it's closer to being possible.

(Eventually, if we don’t come across code that CANNOT be migrated) Drop support for LESS in XWiki completely: most of it has already been converted to CSS at this point.
Note that before reaching this step, we should make sure that all the public extensions have been properly migrated to native CSS with the tool proposed in XWIKI-22670 .