Remove reduntant regex from calls to StringUtils::isUtf8()

I've cautiously moved the regex out of the most used code path.
There is no string that will match that regex check that will not also be
passed by mb_check_encoding.  I think the regex was intended as a shortcut
evaluation, but it is no faster than mb_check_encoding which will often
need to be run anyway.

I think it could just be deleted, but I have limited motivation to
risk introducing a bug to improve performance on old PHP vesions and
unusual configurations, so I've moved it to the fallback code path.

Change-Id: Ie9425cc23ba032e5aff42beeb44cbb1146050452