remove valgrind_stack_change

This is a leftover from when we exposed these APIs to consumers, we
don't actually use this function.

src/debug/valgrind/native.c

@@ -9,10 +9,6 @@ valgrind_stack_id_t valgrind_stack_register(const void *start, const void *end)
   return VALGRIND_STACK_REGISTER(start, end);
 }
 
-void valgrind_stack_change(valgrind_stack_id_t id, const void *start, const void *end) {
-  VALGRIND_STACK_CHANGE(id, start, end);
-}
-
 void valgrind_stack_deregister(valgrind_stack_id_t id) {
   VALGRIND_STACK_DEREGISTER(id);
 }