Fix `mate-netbook` startup delay.

Hi,

I problem was reported with `mate-netbook` that causes significant delay starting panel applets and startup applications.

Problem discussed from the post onward:

  * <http://forums.mate-desktop.org/viewtopic.php?f=4&t=1872#p6627>

Work around documented here:

  * <https://github.com/mate-desktop/mate-netbook/issues/6>

I've also remove `mate-netbook` from the `mate-extras` group. It simply doesn't make sense to install it by default because it change the window manager behaviour that isn't desireable for most users.

Regards, Martin.