libsnapshot: Use SnapshotManager to delete devices.

Diagnosing DM_DEV_REMOVE failures in the test harness is quite
difficult, and it's not clear if failures are spurious or not. Instead
use SnapshotManager's helper function, which can retry on failure, and
will self-diagnose issues on legitimate failures.

Bug: N/A
Test: vts_libsnapshot_test
Change-Id: Ibcaa8406e8b1e8758b99a8e9b58c58d68ed57685
Merged-In: Ibcaa8406e8b1e8758b99a8e9b58c58d68ed57685
(cherry picked from commit e02ef9e9)