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 layout 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.