LibJS: Remove the JS_TRACK_ZOMBIE_CELLS option

This feature had bitrotted somewhat and would trigger errors because
PrimitiveStrings were "destroyed" but because of this mode they were not
removed from the string cache. Even fixing that case running test-js
with the options still failed in more places.

Userland/Libraries/LibTest/JavaScriptTestRunner.h

@@ -116,9 +116,6 @@ static consteval size_t __testjs_last()
 static constexpr auto TOP_LEVEL_TEST_NAME = "__$$TOP_LEVEL$$__";
 extern RefPtr<JS::VM> g_vm;
 extern bool g_collect_on_every_allocation;
-#ifdef JS_TRACK_ZOMBIE_CELLS
-extern bool g_zombify_dead_cells;
-#endif
 extern bool g_run_bytecode;
 extern String g_currently_running_test;
 struct FunctionWithLength {
@@ -291,10 +288,6 @@ inline JSFileResult TestRunner::run_file_test(const String& test_path)
 
     interpreter->heap().set_should_collect_on_every_allocation(g_collect_on_every_allocation);
 
-#ifdef JS_TRACK_ZOMBIE_CELLS
-    interpreter->heap().set_zombify_dead_cells(g_zombify_dead_cells);
-#endif
-
     if (g_run_file) {
         auto result = g_run_file(test_path, *interpreter);
         if (result.is_error() && result.error() == RunFileHookResult::SkipFile) {