fs_mgr: removing the dependency of requiring /vbmeta in fstab for AVB

Remove polling_vbmeta_blk_device() as it tries to get /vbmeta entry
from fstab. Also move the polling of a partition inside read_from_partition()
in fs_mgr_avb_ops.cpp as it's where the reads happen.

Bug: 31264231
Test: Device can boot with AVB

Change-Id: Id717e160b085eca42eb9bc5eb2fa7e658bea3ad6