Revert of Experimental support for RegExp lookbehind. (patchset #18 id:340001 of https://codereview.chromium.org/1418963009/ )

Reason for revert:
gc stress breaks due to string_start_minus_one not being set correctly.

Original issue's description:
> Experimental support for RegExp lookbehind.
>
> R=erikcorry@chromium.org, littledan@chromium.org
> BUG=v8:4545
> LOG=N
>
> Committed: https://crrev.com/37632606bbce1418238b13fd90cb6ef6705871cd
> Cr-Commit-Position: refs/heads/master@{#32029}

TBR=littledan@chromium.org,erikcorry@chromium.org,erikcorry@google.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:4545

Review URL: https://codereview.chromium.org/1451373003

Cr-Commit-Position: refs/heads/master@{#32032}