Remove the timestamp pallet

It was needed for contracts, which has since been removed. We now no longer
need it.

Cargo.lock

@@ -7262,7 +7262,6 @@ dependencies = [
  "sp-keystore",
  "sp-runtime",
  "sp-tendermint",
- "sp-timestamp",
  "substrate-build-script-utils",
  "substrate-frame-rpc-system",
  "validator-sets-pallet",
@@ -7313,7 +7312,6 @@ dependencies = [
  "pallet-balances",
  "pallet-session",
  "pallet-tendermint",
- "pallet-timestamp",
  "pallet-transaction-payment",
  "pallet-transaction-payment-rpc-runtime-api",
  "parity-scale-codec",