Revert of Separate Scope into DeclarationScope and Scope (patchset #13 id:240001 of https://codereview.chromium.org/2209573002/ )

Reason for revert:
[Sheriff] Make leak checker unhappy:
https://build.chromium.org/p/client.v8/builders/V8%20Linux64%20-%20debug/builds/10959

Original issue's description:
> Separate Scope into DeclarationScope and Scope
>
> This reduces peak zone memory usage by ~10% on codeload and ~5% on mandreel.
>
> BUG=v8:5209
>
> Committed: https://crrev.com/2648162dcfff622f8587cea2faa8c3af56456968
> Cr-Commit-Position: refs/heads/master@{#38367}

TBR=marja@chromium.org,mstarzinger@chromium.org,ahaas@chromium.org,adamk@chromium.org,verwaest@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:5209

Review-Url: https://codereview.chromium.org/2212383003
Cr-Commit-Position: refs/heads/master@{#38380}