[heap] Remove some more dead code in MC

src/heap/mark-compact.cc

Index: src/heap/mark-compact.cc
diff --git a/src/heap/mark-compact.cc b/src/heap/mark-compact.cc
index 2e862977812c14d67ddad01a8a76283c7a23bca7..2a464744e20cf120a90196bca277b66e6dac03a8 100644
--- a/src/heap/mark-compact.cc
+++ b/src/heap/mark-compact.cc
@@ -4124,9 +4124,6 @@ void MarkCompactCollector::StartSweepSpaces() {
   heap_->lo_space()->FreeUnmarkedObjects();
 }
 
-Isolate* MarkCompactCollector::isolate() const { return heap_->isolate(); }
-
-
 void MarkCompactCollector::Initialize() {
   MarkCompactMarkingVisitor::Initialize();
   IncrementalMarking::Initialize();