Add `remove_storage` to `AccountUpdate`

[JereSalo]

After #3293

There is an edge case in which we want to remove just the storage of an account without removing the account itself. For now what we are doing is remove the whole account and then insert it again because that's what we can do with the tools that we have. The problem was handled in #3293.
I don't know how trivial it is to insert an empty trie to the account that already exists, I took a look and there doesn't seem to be a method for doing that so it will require more effort than expected.
This addition will improve get_state_transitions() code quality.