UBIFS: remove strange commentary

Remove the following commentary from 'ubifs_file_mmap()':

/* 'generic_file_mmap()' takes care of NOMMU case */

I do not understand what it means, and I could not find anything relater to
NOMMU in 'generic_file_mmap()'.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>