Revert "mm: prioritize tasks holding CMA pages"

This reverts 'commit e1574295
("mm: prioritize tasks holding CMA pages")'

A race is found in the add/delete to the hash list maintained
for exiting tasks. Reverting for now, since this feature itself
is not of much use right now.

Change-Id: Id86839d6d4af35ff9647bc38b65e4aea61f3909c
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>