Remove lessc from autoloader so MW core can load it's own broken compiler

MediaWiki core uses the lessc compiler from http://leafo.net/lessphp .
This compiler requires non-standard Less files which are incompatible
with the compiler used by the Bootstrap extension. It is therefore
necessary to ensure that MW will load its own lessc compiler class
and not the adapter class provided by the Less compiler used by the
Bootstrap extension or else it will not be able to compile its broken
Less files.

Change-Id: I85f3f758ec4603cb983091cd422830080b689ff2