cmd/compile: remove yacc-based parser

- moved yySymType and token constants (L...) to lex.go
- removed oldparser flag and related code
- removed go generate that generated y.go

Fixes #13240.

Change-Id: I2576ec61ee1efe482f2a5132175725c9c02ef977
Reviewed-on: https://go-review.googlesource.com/17176
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: griesemer

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

@@ -627,8 +627,6 @@ var Widthint int
 
 var Widthreg int
 
-var typesw *Node // TODO(gri) remove when yacc-based parser is gone
-
 var nblank *Node
 
 var Funcdepth int32