Part of #1055.
It should have `AllowStates` flag.
Also removed unreachable code: we can't have such situation when
script container is not a transaction in the scope of `CheckWitness`
method because:
1. Blocks have their own implementation of CheckWitness for
internal usage (it's (bc *Blockchain) verifyHeaderWitnesses method).
2. For the outside calls of System.Runtime.CheckWitness interop (e.g.
calls from smart-contract) script container is always a transaction.
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