8275886: G1: remove obsolete comment in HeapRegion::setup_heap_region_size

Reviewed-by: mli, tschatzl

Author: albertnetymk

src/hotspot/share/gc/g1/heapRegion.cpp

@@ -84,8 +84,6 @@ void HeapRegion::setup_heap_region_size(size_t max_heap_size) {
   LogOfHRGrainBytes = region_size_log;
 
   guarantee(GrainBytes == 0, "we should only set it once");
-  // The cast to int is safe, given that we've bounded region_size by
-  // MIN_REGION_SIZE and MAX_REGION_SIZE.
   GrainBytes = region_size;
 
   guarantee(GrainWords == 0, "we should only set it once");