Remove useless open modes

Open modes are ignored if the user doesn't want the file to be created:
http://man7.org/linux/man-pages/man2/open.2.html

Since it's somewhat misleading/potentially indicative of a missing
O_CREAT/O_TMPFILE, we plan to issue a warning on open calls with useless
mode bits in the near future. Because -Werror is used in this project
(thank you!), we need to either make these modes useful, or remove them.

Bug: 64132680
Test: mma. New warnings no longer appear.
Change-Id: I5d9b98152e48135ea716592b28c5c771cc74018b