Revert "[heap] Minor MC: Support ThinString"

This reverts commit 6b29f1cf.

Reason for revert: Breaks many bots. Please no notry:
https://build.chromium.org/p/client.v8/builders/V8%20Linux%20-%20verify%20csa/builds/4512

Original change's description:
> [heap] Minor MC: Support ThinString
> 
> No-try: true
> Bug: chromium:651354, chromium:788113
> Change-Id: I53cc2f04974671982371d1c26c71bd559450a5aa
> Reviewed-on: https://chromium-review.googlesource.com/800176
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Reviewed-by: Hannes Payer <hpayer@chromium.org>
> Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#49752}

TBR=ulan@chromium.org,hpayer@chromium.org,mlippautz@chromium.org

Change-Id: I0a5f47f144ae9c8d2ba279243124fa10ec99e165
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:651354, chromium:788113
Reviewed-on: https://chromium-review.googlesource.com/800491Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49755}