Tombstones no longer exist; now ContractDeposit

[laboon]

They are mentioned here: https://wiki.polkadot.network/docs/build-smart-contracts#contracts-pallet

However, as of Sep 2021 - paritytech/substrate#9669 - these are renamed ContractDeposit.

The PR mainly deletes code and adjusts tests/benchmarks. The biggest change is that the concept of a tombstone is gone. The TombstoneDeposit is renamed to ContractDeposit and is a deposit a deployer needs to deposit into the new contract on top of the ExistentialDeposit (no change to the previous behavior). Please note that seal_terminate can still be used to remove a contract and is the only way to remove a contract and regain the deposit.