ResourceLoaderFileModule: Do not separately cache .less files

ResourceLoaderFileModule: Do not separately cache .less files

We used to have a second level of caching for them – when a module's
cache was invalidated, individual .less files that comprise it would
only be recompiled (with all their @imports) if their mktime changed
(or if $wgResourceLoaderLESSVars changed).

Given that practice has shown that RL modules including .less files
usually just include one file that @imports tons of stuff, and given
that RL already is very careful about invalidating caches, I think
this is unnecessary at best and harmful at worst.

The behavior proved problematic when I tried to implement support for
per-module LESS variables, values of which were language-dependent –
files would not be recompiled for different languages, and
getLessCacheKey() is called several levels deep in code that doesn't
have access to the context in which the module is being compiled.

Change-Id: I667f063507583e4cd3a669e32398d440f1431f7e