mbe: Clean up code with non-optional `NonterminalKind`
Since [rust-lang/rust#128425], the fragment specifier is unconditionally required in all
editions. This means `NonTerminalKind` no longer needs to be optional,
as we can reject this code during the expansion of `macro_rules!` rather
than handling it throughout the code. Do this cleanup here.
[rust-lang/rust#128425]: https://github.com/rust-lang/rust/pull/128425