cmd/compile: cleanup toolstash pacifier from OXFALL removal

Change-Id: Ide7fe6b09247b7a6befbdfc2d6ce5988aa1df323
Reviewed-on: https://go-review.googlesource.com/61131Reviewed-by: Robert Griesemer <gri@golang.org>

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

@@ -563,7 +563,6 @@ const (
 	OCONTINUE // continue
 	ODEFER    // defer Left (Left must be call)
 	OEMPTY    // no-op (empty statement)
-	_         // placeholder to appease toolstash
 	OFALL     // fallthrough
 	OFOR      // for Ninit; Left; Right { Nbody }
 	OFORUNTIL // for Ninit; Left; Right { Nbody } ; test applied after executing body, not before