Description

Container can't be removed if snapshot is deleted first

Steps to reproduce the issue:

1. Run a container and it stops properly
   
2. Remove its snapshot
3. Try to remove this container : ctr c rm test

Describe the results you received:

Describe the results you expected:

container should be removed even if there is no snapshot

Output of containerd --version:

containerd github.com/containerd/containerd v1.2.0-636-g545e79ae.m 545e79a.m

Any other relevant information:

You can delete this container by adding "--keep-snapshot" flag, but it's kind of confusing