[TASK] Remove last LESS file

Remove the last LESS file after the switch to SCSS.

Resolves: #81509
Releases: master
Change-Id: I24138144497c60e018b5612bce3624b631ac6d0b
Reviewed-on: https://review.typo3.org/53165


Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>