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

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}