Remove support for gnome-screensaver and mate-screensaver.

Commit e53afab3 added support for xfce4-screensaver moving the code from xfce4-power-manager.

Please:

• remove support for gnome-screensaver. It's more than a decade without maintenance. Not a very secure option. Not to mention the dispute over the code with Jamie Zawinski.
• remove support for mate-screensaver. It makes a dbus call for mate-power-manager, which conflicts with xfce4-power-manager. Using mate-screensaver with Xfce means suspend/hybernate will not work and an error message will pop up every time user tries to do any of them.