test/images/pets: remove OWNERS to cleanup inactive members

bprashanth hasn't be active since the release of v1.11. Removing them
from test/images/pets/OWNERS would leave mkumatag as the sole approver.

But mkumatag is also an approver for test/images/OWNERS so this commit
removes the test/images/pets/OWNERS completely.

Note: we should try to find more OWNERS for test/images/pets instead,
but this cleanup is a short term solution to avoid the bot suggesting
inactive members for reviews and approval.

Author: nikhita