This reverts commit c3dda0bb.

Reason for revert: Breaks gc stress bots:
https://build.chromium.org/p/client.v8/builders/V8%20Linux%20-%20gc%20stress/builds/14266

Original change's description:
> Enable --harmony-function-tostring by default
>
> Update tests to work with new behavior.
>
> This feature is shipping in Firefox 54, so compatibility risk is low.
>
> R=​littledan@chromium.org, adamk@chromium.org, caitp@igalia.com
> CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel
>
> Bug: v8:4958
> Cq-Include-Trybots: master.tryserver.v8:v8_linux_noi18n_rel_ng
> Change-Id: Ib16d19468cf935f961d7bcd856ebbeb5692d3e61
> Reviewed-on: https://chromium-review.googlesource.com/546941
> Commit-Queue: Josh Wolfe <jwolfe@igalia.com>
> Reviewed-by: Adam Klein <adamk@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#50178}

TBR=adamk@chromium.org,hablich@chromium.org,kozyatinskiy@chromium.org,littledan@chromium.org,caitp@igalia.com,jwolfe@igalia.com

Change-Id: Ie5dd0bd2b97ae6d0126edec6373e48abe0eeb3f0
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:4958
Reviewed-on: https://chromium-review.googlesource.com/832649
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50182}