kill unused deprecated method
kill unused deprecated method

`find_number_of_faces_at_cell` was totally spurious.

RondaStrauch/landlab,cmshobe/landlab,csherwood-usgs/landlab,Carralex/landlab,amandersillinois/landlab,ManuSchmi88/landlab,laijingtao/landlab,csherwood-usgs/landlab,Carralex/landlab,ManuSchmi88/landlab,landlab/landlab,Carralex/landlab,cmshobe/landlab,cmshobe/landlab,landlab/landlab,RondaStrauch/landlab,SiccarPoint/landlab,landlab/landlab,SiccarPoint/landlab,amandersillinois/landlab,RondaStrauch/landlab,ManuSchmi88/landlab,laijingtao/landlab