cmd/internal/gc: remove Node.Builtin

Node.Builtin was occasionally set to 1, but never read.

Change-Id: Ia8a76bccc79b0f211296d50bd419860b13077ba5
Reviewed-on: https://go-review.googlesource.com/6808Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/internal/gc/lex.go

@@ -2502,7 +2502,6 @@ func lexinit() {
 			s1.Def = Nod(ONAME, nil, nil)
 			s1.Def.Sym = s1
 			s1.Def.Etype = uint8(etype)
-			s1.Def.Builtin = 1
 		}
 	}
 
@@ -2630,7 +2629,6 @@ func lexfini() {
 			s.Def = Nod(ONAME, nil, nil)
 			s.Def.Sym = s
 			s.Def.Etype = uint8(etype)
-			s.Def.Builtin = 1
 			s.Origpkg = builtinpkg
 		}
 	}

src/cmd/internal/gc/syntax.go

@@ -37,7 +37,6 @@ type Node struct {
 	Diag           uint8 // already printed error about this
 	Noescape       bool  // func arguments do not escape
 	Nosplit        bool  // func should not execute on separate stack
-	Builtin        uint8 // built-in name, like len or close
 	Nowritebarrier bool  // emit compiler error instead of write barrier
 	Walkdef        uint8
 	Typecheck      uint8