Revert of [regexp] implement /ui to mirror the implementation for /i. (patchset #2 id:20001 of https://codereview.chromium.org/1641613002/ )

Reason for revert:
This test fails:

assertEquals(["as"], /^a[\u017F]/ui.exec("as"));

The reason is that we end up with a character class that is not stand alone, so we do not perform case folding on it correctly (with unicode flag).

Original issue's description:
> [regexp] implement /ui to mirror the implementation for /i.
>
> R=erik.corry@gmail.com, erikcorry@chromium.org
>
> Committed: https://crrev.com/eea1a4c003c559c99bcc9f08aa7eadf931975aad
> Cr-Commit-Position: refs/heads/master@{#33655}

TBR=erik.corry@gmail.com,erikcorry@chromium.org,erikcorry@google.com
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/1661483002

Cr-Commit-Position: refs/heads/master@{#33676}