intel: Drop obsolete comments on MTRR usage

Problem with UMA region allocation was fixed when MTRRs changed to use
memrange implementation.

Change-Id: I420dac30de2836a91596d81f88bb45b46f248532
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/5719
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@google.com>
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>