Removed unused last_block_num from producer_pay

[ericpassmore]

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

reference-contracts/contracts/eosio.system/src/producer_pay.cpp

Lines 27 to 30 in c526479

	// _gstate2.last_block_num is not used anywhere in the system contract code anymore.
	// Although this field is deprecated, we will continue updating it for now until the last_block_num field
	// is eventually completely removed, at which point this line can be removed.
	_gstate2.last_block_num = timestamp;

[ericpassmore]

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

[ericpassmore]

moved to eosnetworkfoundation/eos-system-contracts