Remove TokenKind::InvalidPrefix.

It was added in #123752 to handle some cases involving emoji, but it
isn't necessary because it's always treated the same as
`TokenKind::InvalidIdent`. This commit removes it, which makes things a
little simpler.

Author: nnethercote

crates/parser/src/lexed_str.rs

@@ -183,7 +183,7 @@ impl<'a> Converter<'a> {
                 rustc_lexer::TokenKind::Ident => {
                     SyntaxKind::from_keyword(token_text, self.edition).unwrap_or(IDENT)
                 }
-                rustc_lexer::TokenKind::InvalidPrefix | rustc_lexer::TokenKind::InvalidIdent => {
+                rustc_lexer::TokenKind::InvalidIdent => {
                     err = "Ident contains invalid characters";
                     IDENT
                 }