Revert of Put RegExp js code in strict mode (patchset #2 id:20001 of https://codereview.chromium.org/1776883005/ )

Reason for revert:
Found to break SAP Web IDE, and these semantics are not shipped in any other browser.
Revert to legacy semantics while assessing web compatibility.

BUG=chromium:624318

Original issue's description:
> Put RegExp js code in strict mode
>
> src/js/regexp.js was one of the few files that was left in sloppy
> mode. The ES2017 draft specification requires that writes to
> lastIndex throw when the property is non-writable, and test262
> tests enforce this behavior. This patch puts that file in strict
> mode.
>
> BUG=v8:4504
> R=yangguo@chromium.org
> LOG=Y
>
> Committed: https://crrev.com/80b1b2a45bbd9bf3d08e4e6516acfaaa8f438213
> Cr-Commit-Position: refs/heads/master@{#34801}

TBR=yangguo@chromium.org,adamk@chromium.org

Review-Url: https://codereview.chromium.org/2112713003
Cr-Commit-Position: refs/heads/master@{#37449}