Strip pretty-printed invisible delimiters for type names

[ascjones]

See rust-lang/rust#97076 and rust-lang/rust#96682.

This can happen where TypeInfo is derived for a type generated by a ty expression in a declarative macro. This is parsed as a syn::Type::Group, which is then pretty-printed with the invisible delimiters "/*»*/" and "/*«*/".

These should be stripped, possibly in clean_type_string, but maybe there is a cleaner way to do it.