MAINT: Remove unused futures related methods from asset finder.
MAINT: Remove unused futures related methods from asset finder.

These methods became unused during the course of reworking futures to support
continuous futures.

alphaBenj/zipline,florentchandelier/zipline,humdings/zipline,Scapogo/zipline,florentchandelier/zipline,enigmampc/catalyst,Scapogo/zipline,enigmampc/catalyst,alphaBenj/zipline,quantopian/zipline,quantopian/zipline,humdings/zipline,bartosh/zipline,bartosh/zipline