MAINTAINERS: remove arch/x86/platform/mrst/pmu.*

These files were removed by commit 1a8359e4 ("x86/mid: Remove Intel
Moorestown").

Signed-off-by: Cesar Eduardo Barros <cesarb@cesarb.net>
Cc: Alan Cox <alan@linux.intel.com>
Cc: Len Brown <len.brown@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>