gui: drop deprecated groups

$ desktop-file-validate gui/GameConqueror.desktop
gui/GameConqueror.desktop: hint: value item "GNOME" in key "Categories" in group "Desktop Entry" can be extended with another category among the following categories: GTK
gui/GameConqueror.desktop: warning: value "GNOME;Application;Game;" for key "Categories" in group "Desktop Entry" contains a deprecated value "Application"

Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>