8309286: G1: Remove unused G1HeapRegionAttr::is_valid_gen

Reviewed-by: tschatzl

Author: albertnetymk

src/hotspot/share/gc/g1/g1HeapRegionAttr.hpp

@@ -101,7 +101,6 @@ struct G1HeapRegionAttr {
 #ifdef ASSERT
   bool is_default() const              { return type() == NotInCSet; }
   bool is_valid() const                { return (type() >= Optional && type() < Num); }
-  bool is_valid_gen() const            { return (type() >= Young && type() <= Old); }
 #endif
 };