opengl service: don't override mesa non-drivers in runtime

nixos/modules/hardware/opengl.nix

···
     name = "mesa-drivers+txc-${p.mesa_drivers.version}";
     paths =
       [ p.mesa_drivers
-        p.mesa_drivers.out # mainly for libGL
         (if cfg.s3tcSupport then p.libtxc_dxtn else p.libtxc_dxtn_s2tc)
       ];
   };