Don't create /media

It's obsolete - udisks2 uses /run/media/<username>.

nixos/modules/system/activation/activation-script.nix

···
         mkdir -m 0555 -p /var/empty
       '';
 
-    system.activationScripts.media =
-      ''
-        mkdir -m 0755 -p /media
-      '';
-
     system.activationScripts.usrbinenv =
       ''
         mkdir -m 0755 -p /usr/bin