[format.arg] remove friend declaration from basic_format_arg::handle

[jwakely]

That friend declaration says it's for exposition only, but we don't do the same elsewhere (not even in the adjacent subclauses, e.g. basic_format_args needs access to the private data member of format-arg-store).

We should remove the friend declaration.

[jwakely]

This was left after lwg 3371 removed the other friends.