libsnapshot: Unmap COW devices when deleting snapshots.

When we stopped unmapping snapshots in DeleteSnapshot, this also stopped
unmapping the COW device. Of course, the caller has no way to unmap this
since it's an implementation detail. Therefore unmap it in
DeleteSnapshot.

This scenario can arise after rewriting or removing a snapshot-merge
device.

Bug: 136678799
Test: manual test
Change-Id: I9d4c8bcfbb95b4454edd1cf0853d51b441e895fa