resourceloader: Disable filesystem cache for Less

resourceloader: Disable filesystem cache for Less

Follows-up ea862efa7, which enables use of Less_Cache as part
of the switch from leafo/lessphp to oyejorge/less.php.

Since then 12afb3607 introduced a better caching method in
FileModule::compileLessFile(), which:

* Uses APC where possible, with fallback to main objectcache.
* Verifies both the input file and the indirectly used files have
  not changed before using the cache. The built-in cache system
  only seems to check the root file.

As such, we're incurring the overhead of generating them and storing
on-disk, but raraly ever making use of them. (And even if we do
use it when the APC object first expires, it's risky due to the
lack of indirect file validation).

Bug: T121519
Change-Id: I6e3c66f86a536463488697efc166fb704be0715c