Revert encryption mapping for device wipes.

External storage volumes that were emulated+encrypted needed to have
their encryption mapping removed so that it doesn't try to encrypt the
volume after formatting them.

This just wires through an argument through vold, and assumes that vold
will do the right thing even if there is no encryption mapping set.

Bug: 5017638
Change-Id: I858fae3d12cb415bc34637f520f71220ad9daaad