app: removed unused /dev/ion write permissions

The /dev/ion driver's file operations structure does not specify a
write operation. Granting write is meaningless. This audit statement
has been around since Android Oreo and logs collected from dogfooders
shows that no apps are attempting to open the file with write
permissions.

Bug: 28760354
Test: build
Test: verify no "granted" messages from dogfood devices.
Change-Id: Id4f3540bba8c9f30f9d912f7a7473933be779cbb