Remove MATE from category in dor desktop files.

````
 * QA Notice: This package installs one or more .desktop files that do not
 * pass validation.
 *
 * 	/usr/share/applications/mate-dictionary.desktop: error: value
"MATE;GTK;Office;Dictionary;" for key "Categories" in group "Desktop Entry" contains an
unregistered value "MATE"; values extending the format should start with "X-"
 * 	/usr/share/applications/mate-screenshot.desktop: error: value "GTK;MATE;Utility;" for key
"Categories" in group "Desktop Entry" contains an unregistered value "MATE"; values extending the
format should start with "X-"
 * 	/usr/share/applications/mate-search-tool.desktop: error: value "MATE;GTK;Utility;Core;"
for key "Categories" in group "Desktop Entry" contains an unregistered value "MATE"; values
extending the format should start with "X-"
 * 	/usr/share/applications/mate-system-log.desktop: error: value "GTK;MATE;System;Monitor;"
for key "Categories" in group "Desktop Entry" contains an unregistered value "MATE"; values
extending the format should start with "X-"
 *
````