Remove obsolete code related to patchoat.

Remove code that was opening boot oat files from /system/ as
read-only for failed patchoat. Patchoat has been removed.

Test: m test-art-host
Test: testrunner.py --host --optimizing
Bug: 77856493
Change-Id: I0c2b804c28bb34a749218d75875a4fbee9238b7f