Remove create_chars_child_column utility

[davidwendt]

Description

Removes the cudf::strings::detail::create_chars_child_column utility. This is not longer needed or used.
Removing it helps prevent inadvertently using it to wrap chars data with a cudf column.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[mhaseeb123]

Looks good to me. The branch seems to be out of date with the base branch which may be fast forwarded.

[davidwendt]

/merge