gl: remove duplicate build tag
gl: remove duplicate build tag

Change-Id: I7dcb0b3b946cfae86eb6eed509ad96543213c409
Reviewed-on: https://go-review.googlesource.com/108335
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/mobile,golang/mobile,golang/mobile,golang/mobile