Revert of [Parser] Introduce AstStringConstants to share constants across AstValueFactory (patchset #4 id:80001 of https://codereview.chromium.org/2630343002/ )

Reason for revert:
Seems to break modules-namespace2 on gcstress.

Original issue's description:
> [Parser] Introduce AstStringConstants to share constants across AstValueFactory
>
> Creates an AstStringConstants container which pre-initializes the
> string constants used by AstValueFactory. This ensures that all
> AstValueFactories will produce the same AstValue objects for constants,
> and so they can be used by the BytecodeGenerator without having to pass
> the AstValueFactory to it, enabling construction off-thread.
>
> BUG=v8:5203
>
> Review-Url: https://codereview.chromium.org/2630343002
> Cr-Commit-Position: refs/heads/master@{#42381}
> Committed: https://chromium.googlesource.com/v8/v8/+/d611496b8ed30af787d8668f96b400617c858508

TBR=ahaas@chromium.org,marja@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:5203

Review-Url: https://codereview.chromium.org/2638783002
Cr-Commit-Position: refs/heads/master@{#42382}