Revert of Internalize strings in-place (patchset #20 id:380001 of https://codereview.chromium.org/2549773002/ )

Reason for revert:
Blocks roll, ASan detects leaking ExternalStrings.

Original issue's description:
> Internalize strings in-place (reland^2)
>
> using newly introduced ThinStrings, which store a pointer to the actual,
> internalized string they represent.
>
> BUG=v8:4520
>
> (Previously landed as #42168 / af51befe)
> (Previously landed as #42193 / 4c699e34)
>
> Review-Url: https://codereview.chromium.org/2549773002
> Cr-Commit-Position: refs/heads/master@{#42235}
> Committed: https://chromium.googlesource.com/v8/v8/+/ec45e6ed2e11698c713e664b1510bc31bcdbbdba

TBR=ishell@chromium.org,hpayer@chromium.org,bmeurer@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:4520

Review-Url: https://codereview.chromium.org/2626893005
Cr-Commit-Position: refs/heads/master@{#42271}