[heap] Remove unused IsMarkingIncomplete().

Change-Id: I5523f61627cab0ff0b921e51038396c642dad017
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3351784Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78424}

src/heap/incremental-marking.h

@@ -114,8 +114,6 @@ class V8_EXPORT_PRIVATE IncrementalMarking final {
 
   inline bool IsMarking() const { return state() >= MARKING; }
 
-  inline bool IsMarkingIncomplete() const { return state() == MARKING; }
-
   inline bool IsComplete() const { return state() == COMPLETE; }
 
   inline bool IsReadyToOverApproximateWeakClosure() const {