[POWERPC] Remove bogus comment about page_is_ram

arch/powerpc/mm/mem.c states that page_is_ram is called by the code that
implements /dev/mem, which isn't true.  Remove the comment.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>