Revert of [regexp] Support unicode capture names in non-unicode patterns (patchset #3 id:40001 of https://codereview.chromium.org/2791163003/ )

Reason for revert:
The decision for the specification was to not have this syntax, and instead the syntax before this patch.

Original issue's description:
> [regexp] Support unicode capture names in non-unicode patterns
>
> This ensures that capture names containing surrogate pairs are parsed
> correctly even in non-unicode RegExp patterns by introducing a new
> scanning mode which unconditionally combines surrogate pairs.
>
> BUG=v8:5437,v8:6192
>
> Review-Url: https://codereview.chromium.org/2791163003
> Cr-Commit-Position: refs/heads/master@{#44466}
> Committed: https://chromium.googlesource.com/v8/v8/+/a8651c5671dd6e41595ffb438e7ea013082f2d38

R=yangguo@chromium.org,jgruber@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=v8:5437,v8:6192

Review-Url: https://codereview.chromium.org/2859933003
Cr-Commit-Position: refs/heads/master@{#45088}