Remove defunct values from mainnet's context

We have found 283K /contracts/index/**/spendable files in today’s mainnet context which are defunct for long since Babylone upgrade. They should be removed in a future migration.

We are working on context schema which summarizes the shape of Tezos context for further optimization of context I/O. It is based on Storage_description which defines similar schema but for RPC. The schema itself is still at PoC, but schema checking of today’s Tezos context revealed there exist 283K spendable files out of it. They should have been removed at Babylon upgrade, but it only removed those of the originated contracts. The spendable files of the implicit accounts were left not removed. It is not urgent but we think a future protocol upgrade should clean them away.