LibJS: Put zombie cell tracking code behind a compile-time flag

Since this is a debug-only feature, let's not have it impact GC marking
performance when you don't need it.

Userland/Libraries/LibJS/Runtime/Shape.cpp

@@ -238,9 +238,11 @@ FLATTEN void Shape::add_property_without_transition(PropertyName const& property
     add_property_without_transition(property_name.to_string_or_symbol(), attributes);
 }
 
+#ifdef JS_TRACK_ZOMBIE_CELLS
 void Shape::did_become_zombie()
 {
     revoke_weak_ptrs();
 }
+#endif
 
 }