adb remount test: limit override_creds check to kernels before 6.12

Since we have a new way of supporting remounts which does not need
override_creds, we removed override_creds support from 6.12 and beyond.

Remove check for override_creds in 6.12 and later kernels.

Bug: 406813943
Test: atest adb-remount.sh
Change-Id: I9a978b509b382334164834dcd53be3249ccc976c