Revert "[heap] Fix incremental wrapper tracing toggle"

This reverts commit 9e839fce.

Reason for revert: Wrong fix as we are not allowed to cache wrappers.

Original change's description:
> [heap] Fix incremental wrapper tracing toggle
> 
> The flag is always on and support for turning it off is broken with
> conservative barriers.
> 
> Bug: 
> Change-Id: I1ff548f95d220bf0fcb6df7a1bf5f8a342163696
> Reviewed-on: https://chromium-review.googlesource.com/624494
> Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#47482}

TBR=ulan@chromium.org,mlippautz@chromium.org

Change-Id: I90bc547a88cb8220c7261c607ef359df38e3bdf2
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/623868Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47494}

src/heap/incremental-marking.cc

@@ -536,7 +536,7 @@ void IncrementalMarking::StartMarking() {
 
   SetState(MARKING);
 
-  if (FLAG_incremental_marking_wrappers) {
+  {
     TRACE_GC(heap()->tracer(),
              GCTracer::Scope::MC_INCREMENTAL_WRAPPER_PROLOGUE);
     heap_->local_embedder_heap_tracer()->TracePrologue();