Revert of Stage sloppy let

Revert of Stage sloppy let (patchset #7 id:120001 of https://codereview.chromium.org/1327483002/ )

Reason for revert:
Fails a test262 test with --always-opt.

Original issue's description:
> Stage sloppy let
>
> Move the --harmony-sloppy-let flag to staging for further testing, and
> update test262 for the new passing tests. Also increase the strictness
> of the parser, even in sloppy mode, to disallow "new legacy compat" for
>
>   for (let x = 5 in {}) {}
>
> which is now a SyntaxError.
>
> BUG=v8:3305
> LOG=Y
> R=adamk
>
> Committed: https://crrev.com/07bc0117be8dc9e63ec14d5f9645c483d60a1bec
> Cr-Commit-Position: refs/heads/master@{#30515}

TBR=adamk@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:3305
Committed: https://crrev.com/d1fa7bccc80ce67ab52cbbd3daadeeab47a365c9
Cr-Commit-Position: refs/heads/master@{#30518}

[Dan Ehrenberg, 2015-09-01 17:36:26]

Created Revert of Stage sloppy let

[commit-bot: I haz the power, 2015-09-01 17:36:36]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1324033002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1324033002/1

[commit-bot: I haz the power, 2015-09-01 17:36:57]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-01 17:37:16]

Patchset 1 (id:??) landed as
https://crrev.com/d1fa7bccc80ce67ab52cbbd3daadeeab47a365c9
Cr-Commit-Position: refs/heads/master@{#30518}