remove unused variable _consecutive_production_enabled

[secp256]

No description provided.

[abitmore]

Note: related startup option was removed in this commit: 26007bb

[oxarbitrage]

thanks @zhuliting for the work and @abitmore for adding the commit where the code for the option was removed.

[abitmore]

Perhaps we can remove these as well:

bitshares-core/libraries/plugins/witness/include/graphene/witness/witness.hpp

Line 44 in 7e60b16

consecutive = 7,

bitshares-core/libraries/plugins/witness/witness.cpp

Lines 199 to 201 in 7e60b16

	case block_production_condition::consecutive:
	elog("Not producing block because the last block was generated by the same witness.\nThis node is probably disconnected from the network so block production has been disabled.\nDisable this check with --allow-consecutive option.");
	break;