proprietary-files.txt: remove unneeded powervr.ini

We inherited this from somewhere a long time ago.  Strictly, it's been
unneeded for a while now, but we kept it because it was needed for
working hwcomposer.  Now that the kernel's been fixed to eliminate the
need for that workaround, we can just get rid of it entirely.

Change-Id: I38525e31cc05182ead7fedd4de6987912e1f5856