This reverts commit 5863c0b6.

Reason for revert: Breaks GPU bots (https://ci.chromium.org/p/v8/builders/luci.v8.ci/Mac%20V8%20FYI%20Release%20(Intel)/1648)

Original change's description:
> [heap] Added External Strings to external memory accounting.
> 
> Bug: chromium:845409
> Change-Id: I3fe2b294f6e038d77787cf0870d244ba7cc20550
> Reviewed-on: https://chromium-review.googlesource.com/1118164
> Commit-Queue: Rodrigo Bruno <rfbpb@google.com>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#54110}

TBR=ulan@chromium.org,hpayer@chromium.org,mlippautz@chromium.org,rfbpb@google.com

Change-Id: I77787df6aef2efec95c79714d1baf43feeb8a27e
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:845409
Reviewed-on: https://chromium-review.googlesource.com/1120265Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Rodrigo Bruno <rfbpb@google.com>
Cr-Commit-Position: refs/heads/master@{#54114}