libsnapshot: don't kill the daemon for legacy vab snapshots

If partitions are mounted off the daemon, there is no need
to kill if the tests are being run for legacy vab snapshots.

This also removes vabc_legacy_test as it is no longer required.

Bug: 331053511
Test: vab_legacy_test, vts_libsnapshot_test on Pixel - No flake observed
with 10 iterations
Change-Id: Ie8b29fef77948d23d920c19d816376290cf2fed9
Signed-off-by: Akilesh Kailash <akailash@google.com>