Revert of Add CHECKs to verify that we never finalize stale copies of external...

Revert of Add CHECKs to verify that we never finalize stale copies of external strings (patchset #1 id:1 of https://codereview.chromium.org/1160253010/)

Reason for revert:
not needed anymore

Original issue's description:
> Add CHECKs to verify that we never finalize stale copies of external strings
>
> BUG=none
> R=ulan@chromium.org
> LOG=n
>
> Committed: https://crrev.com/ca2f8d811ca08783e38ce5dd42d38a6b6a7b876f
> Cr-Commit-Position: refs/heads/master@{#28811}

TBR=ulan@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=none

Review URL: https://codereview.chromium.org/1178703008

Cr-Commit-Position: refs/heads/master@{#28932}

src/heap/mark-compact.cc

@@ -1679,20 +1679,13 @@ class StringTableCleaner : public ObjectVisitor {
       if (o->IsHeapObject() &&
           Marking::IsWhite(Marking::MarkBitFrom(HeapObject::cast(o)))) {
         if (finalize_external_strings) {
-          // We must never finalize a string that was copied (has a forwarding
-          // map).
-          CHECK(o->IsExternalString());
+          DCHECK(o->IsExternalString());
           heap_->FinalizeExternalString(String::cast(*p));
         } else {
           pointers_removed_++;
         }
         // Set the entry to the_hole_value (as deleted).
         *p = heap_->the_hole_value();
-      } else {
-        // Anything in the external string table has to be either a string or
-        // the hole.
-        CHECK(!finalize_external_strings || o->IsExternalString() ||
-              o->IsTheHole());
       }
     }
   }