[heap] No need for snapshotting protocol on ThinStrings

ThinStrings are not transitioned to other string representations, so we
shouldn't need the snapshotting protocol for them.

Change-Id: I17cee1a4171b10c441a005ac29bd232a0a065207
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3852489Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82727}

src/heap/concurrent-marking.cc

@@ -306,11 +306,6 @@ class YoungGenerationConcurrentMarkingVisitor final
                                                                    object);
   }
 
-  int VisitThinString(Map map, ThinString object) {
-    return ConcurrentMarkingVisitorUtility::VisitFullyWithSnapshot(this, map,
-                                                                   object);
-  }
-
   int VisitSeqOneByteString(Map map, SeqOneByteString object) {
     if (!ShouldVisit(object)) return 0;
     return SeqOneByteString::SizeFor(object.length(kAcquireLoad));
@@ -443,11 +438,6 @@ class ConcurrentMarkingVisitor final
                                                                    object);
   }
 
-  int VisitThinString(Map map, ThinString object) {
-    return ConcurrentMarkingVisitorUtility::VisitFullyWithSnapshot(this, map,
-                                                                   object);
-  }
-
   int VisitSeqOneByteString(Map map, SeqOneByteString object) {
     if (!ShouldVisit(object)) return 0;
     VisitMapPointer(object);