- 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/17176Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>