remove unused function tree_to_list
remove unused function tree_to_list

MSeifert04/astropy,saimn/astropy,dhomeier/astropy,saimn/astropy,astropy/astropy,lpsinger/astropy,larrybradley/astropy,mhvk/astropy,stargaser/astropy,StuartLittlefair/astropy,aleksandr-bakanov/astropy,dhomeier/astropy,astropy/astropy,pllim/astropy,aleksandr-bakanov/astropy,mhvk/astropy,stargaser/astropy,saimn/astropy,astropy/astropy,mhvk/astropy,bsipocz/astropy,astropy/astropy,dhomeier/astropy,aleksandr-bakanov/astropy,dhomeier/astropy,larrybradley/astropy,bsipocz/astropy,dhomeier/astropy,StuartLittlefair/astropy,MSeifert04/astropy,lpsinger/astropy,StuartLittlefair/astropy,aleksandr-bakanov/astropy,astropy/astropy,StuartLittlefair/astropy,stargaser/astropy,lpsinger/astropy,pllim/astropy,bsipocz/astropy,MSeifert04/astropy,stargaser/astropy,bsipocz/astropy,larrybradley/astropy,larrybradley/astropy,lpsinger/astropy,larrybradley/astropy,mhvk/astropy,saimn/astropy,lpsinger/astropy,mhvk/astropy,pllim/astropy,saimn/astropy,pllim/astropy,pllim/astropy,StuartLittlefair/astropy,MSeifert04/astropy