Rollup merge of #142657 - tgross35:nonoptional-fragment-specifiers-cleanup, r=petrochenkov

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

compiler/rustc_ast/src/token.rs

@@ -1085,6 +1085,7 @@ pub enum NtExprKind {
     Expr2021 { inferred: bool },
 }
 
+/// A macro nonterminal, known in documentation as a fragment specifier.
 #[derive(Debug, Copy, Clone, PartialEq, Eq, Encodable, Decodable, Hash, HashStable_Generic)]
 pub enum NonterminalKind {
     Item,