gl: remove duplicate build tag

Change-Id: I7dcb0b3b946cfae86eb6eed509ad96543213c409
Reviewed-on: https://go-review.googlesource.com/108335
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/gl/work.go b/gl/work.go
index 470f57e..3a8bacf 100644
--- a/gl/work.go
+++ b/gl/work.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build darwin linux openbsd openbsd
+// +build darwin linux openbsd
 
 package gl