cmd/compile: remove unneeded error message cleanup pass

This code used to be necessary because of the error messages generated
by the YACC-based parser, but they're no longer relevant under the new
recursive descent parser:

  - LBRACE no longer exists, so "{ or {" can never occur.

  - The parser never generates error messages about "@" or "?" now
    (except in import sections, where they're actually legitimate).

  - The s/LLITERAL/litbuf/ substitution is handled in p.syntax_error.

Change-Id: Id39f747e4aa492c5830d14a47b161920bd4589ad
Reviewed-on: https://go-review.googlesource.com/17690Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/gc/parser.go

@@ -142,7 +142,6 @@ func (p *parser) syntax_error(msg string) {
 	var tok string
 	switch p.tok {
 	case LLITERAL:
-		// this is also done in Yyerror but it's cleaner to do it here
 		tok = litbuf
 	case LNAME:
 		if p.sym_ != nil && p.sym_.Name != "" {

src/cmd/compile/internal/gc/subr.go

@@ -134,16 +134,6 @@ func Yyerror(format string, args ...interface{}) {
 			return
 		}
 
-		// The grammar has { and LBRACE but both show up as {.
-		// Rewrite syntax error referring to "{ or {" to say just "{".
-		// The grammar has ? and @ but only for reading imports.
-		// Silence them in ordinary errors.
-		msg = strings.Replace(msg, "{ or {", "{", -1)
-		msg = strings.Replace(msg, " or ?", "", -1)
-		msg = strings.Replace(msg, " or @", "", -1)
-
-		msg = strings.Replace(msg, "LLITERAL", litbuf, -1)
-
 		yyerrorl(int(lexlineno), "%s", msg)
 		return
 	}