prune: Error if --static-deltas-only without --delete-commit
The original changes here apparently had the *idea* that `--static-deltas-only`
would be useful in general, but we never implemented that. The current
situation where it's ignored unless `--delete-commit` is specified is
very misleading and I can easily see it leading to data loss for people.
Let's error out until we have a chance to make it actually useful.
Related: https://github.com/ostreedev/ostree/issues/1479
Closes: #1482
Approved by: giuseppe