remount: Simplify disable verity logic

We are calling avb_user_verity_set() _for each_ fstab entry in order to
disable verity. This is an artifact from back when each partition has
its own verity flag (VB1.0). Since AVB, the verity flag in vbmeta
affects all dm-verity device, thus we only need to call
avb_user_verity_set() once.

Bug: 241688845
Test: adb-remount-test
Change-Id: Ie5b788e2d34f83152228db62f84d26a32e2b26e0