first_stage_mount: Remove "overlay" hack from InitRequiredDevices()

GetDmVerityDevices() should filter out overlayfs fstab entries in the
first place, so InitRequiredDevices() don't need to filter out overlayfs
pseudo device names.

Bug: 188862155
Test: Boot to normal with overlayfs mount entries in first stage fstab
Change-Id: I0ac8b7ac0f21daa0c191580d9349adf217854864
Merged-In: I0ac8b7ac0f21daa0c191580d9349adf217854864
(cherry picked from commit 87290f8e)