Revert of Group lexical context variables for faster look up. (patchset #2 id:20001 of https://codereview.chromium.org/1281883002/ )

Reason for revert:
This performance hack is no longer necessary.

Original issue's description:
> Group lexical context variables for faster look up.
>
> Currently, looking up a lexical context variable requires looking up
> the variable name and then checking its mode. This can be a bottleneck
> in Runtime_DeclareGlobals, even when no lexical context variables are
> declared.
>
> R=rossberg@chromium.org
> BUG=crbug:517778
> LOG=N
>
> Committed: https://crrev.com/a45ed17bb6aca02e940f13bbf456d660cccc86ae
> Cr-Commit-Position: refs/heads/master@{#30075}

TBR=rossberg@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=crbug:517778

Review URL: https://codereview.chromium.org/1290053002

Cr-Commit-Position: refs/heads/master@{#30145}