Remove useless mutation
The MutateHeadTransition mutation was exercising the case of a rogue close transaction trying to steal some commits. The attack could have been the following: * the head is initialized * some members commit * a rogue member creates a close transaction consuming all the commits and stealing the corresponding funds. To explore this scenario, we crafted a close transaction which passes L2 phase validation and is balanced. So successful in a way. But we realize that there's no way one could create an open state (which is needed for a valid close) without first having spent the commits. Said differently, the pre-requisite for this mutation was to be in an impossible state. Our conclusion is that this mutation does not make sense anymore. Hence, we remove it.
