fs_mgr: remove same partition check for vbmeta

With chained vbmeta and especially its use with dynamic partitions,
this check is no longer always true.  A concrete example is the
chained vbmeta_system partition that contains the metadata for /system
and /product_services for mainline devices.

Test: blueline_mainline boots with AVB enabled
Change-Id: Ib4115831bb3bb49bf6d2a908df59ba32f8405de7