This reverts commit 28930128.

Reason for revert: GC stress failures:
https://build.chromium.org/p/client.v8/builders/V8%20Mac%20GC%20Stress/builds/12958

Original change's description:
> [runtime] Keep FAST_SLOPPY_ARGUMENTS packed
> 
> With this CL SloppyArguments immediately go to dictionary elements on 
> deletion, keeping the arguments backing store packed.
> 
> Bug: v8:6251
> Change-Id: I2afa4fb5f0af9942eee0a1606942f5f289539330
> Reviewed-on: https://chromium-review.googlesource.com/480379
> Commit-Queue: Camillo Bruni <cbruni@chromium.org>
> Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#44857}

TBR=jkummerow@chromium.org,cbruni@chromium.org,v8-reviews@googlegroups.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Change-Id: I9482bf693a745d1301d068869ddae39f11143827
Reviewed-on: https://chromium-review.googlesource.com/486885Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44863}