Remove useless open mode

Open modes are ignored if the user doesn't want the file to be crated:
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. Since -Werror is used in this project
(thank you!), we need to either make the mode useful, or remove it.

Bug: 64132680
Test: mma. New warning no longer appears.
Change-Id: I2ef32732956e2ef7ee46af2f8575b3bc7c0c5655