Investigate gas consumption in delegation that empties contract

!5506 (merged) has uncovered that in test_self_delegation_emptying_contract in proto_alpha/lib_protocol/test/integration/consensus/test_delegation.ml, the delegation consumes more than 1000 gas (the constant gas cost required of every manager operation). Additional gas seems to be consumed when spending the fees because this empties the contract. In particular, in Contract_storage.delete, the call to Storage.Contract.Code.remove for an Implicit contract looks suspicious.