Revert of Internalize strings in-place (patchset #17 id:320001 of https://codereview.chromium.org/2549773002/ )

Reason for revert:
blocks roll, see: https://codereview.chromium.org/2628733002/

Debug mode runs into an Abort("External string expected, but not found").

Original issue's description:
> Internalize strings in-place (reland)
>
> using newly introduced ThinStrings, which store a pointer to the actual,
> internalized string they represent.
>
> BUG=v8:4520
>
> (Previously landed as #42168 / af51befe.
>
> Review-Url: https://codereview.chromium.org/2549773002
> Cr-Commit-Position: refs/heads/master@{#42193}
> Committed: https://chromium.googlesource.com/v8/v8/+/4c699e349a4986b28574b3a51e8780e3a3d067b1

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/2625073002
Cr-Commit-Position: refs/heads/master@{#42212}