Remove NtVis.

We now use invisible delimiters for expanded `vis` fragments, instead of
`Token::Interpolated`.

compiler/rustc_ast/src/tokenstream.rs

@@ -472,7 +472,6 @@ impl TokenStream {
             Nonterminal::NtTy(ty) => TokenStream::from_ast(ty),
             Nonterminal::NtMeta(attr) => TokenStream::from_ast(attr),
             Nonterminal::NtPath(path) => TokenStream::from_ast(path),
-            Nonterminal::NtVis(vis) => TokenStream::from_ast(vis),
             Nonterminal::NtExpr(expr) | Nonterminal::NtLiteral(expr) => TokenStream::from_ast(expr),
         }
     }