Don't auto-apply the lessphp filter to all LESS files

We shouldn't auto apply the lessphp filter to all LESS files, since we don't know the organisation of the parent application.

If the parent application uses LESS files but depends on a certain order for compilation (e.g. via `@import`) the blanket application of this filter could cause a compilation error (e.g. if a LESS variable used in one file is defined in another).

Since we are already explicitly specifying the filter to be used in `@bootstrap_less` we can safely remove the blanket filter application for all LESS files.