libsnapshot: Don't force-unmap in DeleteSnapshot().

It is hard to re-use this function in the merge code when it forcefully
unmaps the snapshot, because the snapshot may have been rewritten to be
a dm-linear device. Instead, leave the decision up to the caller.

Bug: 136678799
Test: libsnapshot_test gtest
Change-Id: I03c027c0781696885a5a5654d3049287cc16ecd0