Remove MATE category from dot desktop files.

Make my QA payment for gentoo.

 ````
* QA Notice: This package installs one or more .desktop files that do not
 * pass validation.
 * 
 * 	/usr/share/applications/mate-network.desktop: error: value ";GTK;System;Settings;X-MATE-NetworkSettings;" for key "Categories" in group "Desktop Entry" contains an unregistered value ""; values extending the format should start with "X-"
 * 	/usr/share/applications/mate-services.desktop: error: value ";GTK;System;Settings;" for key "Categories" in group "Desktop Entry" contains an unregistered value ""; values extending the format should start with "X-"
 * 	/usr/share/applications/mate-shares.desktop: error: value ";GTK;System;Settings;X-MATE-NetworkSettings;" for key "Categories" in group "Desktop Entry" contains an unregistered value ""; values extending the format should start with "X-"
 * 	/usr/share/applications/mate-time.desktop: error: value ";GTK;System;Settings;" for key "Categories" in group "Desktop Entry" contains an unregistered value ""; values extending the format should start with "X-"
 * 	/usr/share/applications/mate-users.desktop: error: value ";GTK;System;Settings;" for key "Categories" in group "Desktop Entry" contains an unregistered value ""; values extending the format should start with "X-"
````