Revert of Internalize strings in-place

src/heap/objects-visiting.cc

Index: src/heap/objects-visiting.cc
diff --git a/src/heap/objects-visiting.cc b/src/heap/objects-visiting.cc
index a2651bfb987b2cc2117fa0173ac507642fe8cc66..146aa58675eaf96f128a6dc002b4990b1e1fce47 100644
--- a/src/heap/objects-visiting.cc
+++ b/src/heap/objects-visiting.cc
@@ -41,9 +41,6 @@ StaticVisitorBase::VisitorId StaticVisitorBase::GetVisitorId(
       case kExternalStringTag:
         return GetVisitorIdForSize(kVisitDataObject, kVisitDataObjectGeneric,
                                    instance_size, has_unboxed_fields);
-
-      case kThinStringTag:
-        return kVisitThinString;
     }
     UNREACHABLE();
   }