snapuserd: Remove legacy dm-snapshot based snapshot and snapshot-merge

Clean up all the legacy dm-snapshot based approach.

This will continue to maintain backward compatibility with vendor
partition on Android S.

Bug: 304829384
Test: OTA on Pixel
th - OTA from Android S to TOT with vendor on S
Change-Id: Id9263be881dd1a06923cd0ace007f1c027e6969d
Signed-off-by: Akilesh Kailash <akailash@google.com>