fs_mgr: overlay drop redundant check

fs_mgr_update_verity_state() provides the needed result, we do not
need to check verity state manually.

Caveat:

The open-coded verity check is not 100% redundant, as it ensures
that if /vendor is not mounted, where the device mount table resides
to aid in a correct enumeration of all verity-enabled mount points,
that /system is not accidentally overlay mounted on a verity checked
volume.  This is a unlikely corner condition.  A fix for this
condition in the future is to instead migrate this redundant
checking into fs_mgr_update_verity_state() since system is
considered ever present and can be blindly performed without the
required /system or / mount point entries in fstab.  Note that
the overlay logic is #ifdef'd out on user builds, so it is not
really a security or reliability issue on a release build.

Test: manual
Bug: 109821005
Change-Id: Ib4a7f9438b2a3cb008e263605a7a7647737c40f2