Replace `string_split` impl with `string_split_noregexp`

[toddjonker]

The current implementation of string_split accidentally supports splitting on a regular expression.

Its sibling string_split_noregexp was introduced to enable migration.

At this point we should just replace the implementation of the former, and remove the latter.

[cnaples79]

I've opened a PR for this! Let me know if I need to make any changes.

[toddjonker]

Thank you! Just sent a review.