Revert of Internalize strings in-place (patchset #16 id:300001 of https://codereview.chromium.org/2549773002/ )

Reason for revert:
gc stress failures:
https://build.chromium.org/p/client.v8/builders/V8%20Linux%20-%20gc%20stress/builds/8024

Original issue's description:
> Internalize strings in-place
>
> using newly introduced ThinStrings, which store a pointer to the actual,
> internalized string they represent.
>
> BUG=v8:4520
>
> Review-Url: https://codereview.chromium.org/2549773002
> Cr-Commit-Position: refs/heads/master@{#42168}
> Committed: https://chromium.googlesource.com/v8/v8/+/af51befe694fe039db3554d4b9165f7d6baceb77

TBR=ishell@chromium.org,hpayer@chromium.org,bmeurer@chromium.org,jkummerow@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/2621913002
Cr-Commit-Position: refs/heads/master@{#42170}