Removed unused last_block_num from producer_pay

[ericpassmore]

Transferred from AntelopeIO/reference-contracts#111 original author ericpassmore

Remove this unused update. This has been here since 2019.

https://github.com/eosnetworkfoundation/eos-system-contracts/blob/aaa54a39c90f2e032b84ee94dc3574d63a42daa7/contracts/eosio.system/src/producer_pay.cpp#L27-L30

[ericpassmore]

This also shows up in the ABI for tests/test_contracts/old_versions, and assume we would keep the old versions unchanged.