Remove String::get_Chars workaround
Remove String::get_Chars workaround

CoreLib now has a workaround applied that lets us compile this method
just fine and this is giving errors about duplicate symbols.

sandreenko/corert,schellap/corert,schellap/corert,krytarowski/corert,gregkalapos/corert,tijoytom/corert,mjp41/corert,tijoytom/corert,botaberg/corert,schellap/corert,botaberg/corert,mjp41/corert,krytarowski/corert,shrah/corert,mjp41/corert,gregkalapos/corert,manu-silicon/corert,manu-silicon/corert,shrah/corert,schellap/corert,kyulee1/corert,yizhang82/corert,manu-silicon/corert,gregkalapos/corert,kyulee1/corert,tijoytom/corert,shrah/corert,yizhang82/corert,botaberg/corert,manu-silicon/corert,botaberg/corert,tijoytom/corert,krytarowski/corert,sandreenko/corert,gregkalapos/corert,yizhang82/corert,mjp41/corert,shrah/corert,kyulee1/corert,sandreenko/corert,krytarowski/corert,schellap/corert,mjp41/corert,sandreenko/corert,manu-silicon/corert,kyulee1/corert,yizhang82/corert