.gitignore: don't ignore y.output

We no longer use yacc, and we shouldn't have any y.output files.

Change-Id: I045671b6aef3f53c3cfe068b0c14a4871689c13e
Reviewed-on: https://go-review.googlesource.com/22161
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

.gitignore

@@ -24,7 +24,6 @@ doc/articles/wiki/*.bin
 misc/cgo/life/run.out
 misc/cgo/stdio/run.out
 misc/cgo/testso/main
-src/cmd/**/y.output
 src/cmd/cgo/zdefaultcc.go
 src/cmd/go/zdefaultcc.go
 src/cmd/internal/obj/zbootstrap.go