Remove reference to `FunctionQuantity.to` in docs.
Remove reference to `FunctionQuantity.to` in docs.

Since that is no longer overridden.

bsipocz/astropy,tbabej/astropy,aleksandr-bakanov/astropy,AustereCuriosity/astropy,pllim/astropy,tbabej/astropy,MSeifert04/astropy,lpsinger/astropy,joergdietrich/astropy,pllim/astropy,dhomeier/astropy,saimn/astropy,bsipocz/astropy,dhomeier/astropy,bsipocz/astropy,larrybradley/astropy,lpsinger/astropy,DougBurke/astropy,AustereCuriosity/astropy,lpsinger/astropy,astropy/astropy,funbaker/astropy,larrybradley/astropy,mhvk/astropy,mhvk/astropy,dhomeier/astropy,AustereCuriosity/astropy,StuartLittlefair/astropy,DougBurke/astropy,mhvk/astropy,pllim/astropy,funbaker/astropy,kelle/astropy,MSeifert04/astropy,kelle/astropy,StuartLittlefair/astropy,larrybradley/astropy,joergdietrich/astropy,larrybradley/astropy,StuartLittlefair/astropy,StuartLittlefair/astropy,stargaser/astropy,kelle/astropy,pllim/astropy,aleksandr-bakanov/astropy,joergdietrich/astropy,saimn/astropy,funbaker/astropy,astropy/astropy,joergdietrich/astropy,joergdietrich/astropy,tbabej/astropy,DougBurke/astropy,AustereCuriosity/astropy,larrybradley/astropy,StuartLittlefair/astropy,kelle/astropy,lpsinger/astropy,DougBurke/astropy,aleksandr-bakanov/astropy,astropy/astropy,astropy/astropy,aleksandr-bakanov/astropy,mhvk/astropy,funbaker/astropy,saimn/astropy,saimn/astropy,stargaser/astropy,MSeifert04/astropy,dhomeier/astropy,MSeifert04/astropy,AustereCuriosity/astropy,mhvk/astropy,saimn/astropy,dhomeier/astropy,bsipocz/astropy,stargaser/astropy,tbabej/astropy,astropy/astropy,kelle/astropy,stargaser/astropy,lpsinger/astropy,tbabej/astropy,pllim/astropy