Remove maintainership

With this patch I remove myself as a maintainer for all packages I
currently maintain.

This is due the fact that I will be basically off the grid from May 2018
until early 2019, as I will be on a trip through north america.

I will revert this patch as soon as I'm back, as I plan to continue
contributing to nixpkgs then.
But as I cannot maintain anything during that time, I'd like to get this
patch merged.

Signed-off-by: Matthias Beyer <mail@beyermatthias.de>

pkgs/desktops/xfce/panel-plugins/xfce4-mailwatch-plugin.nix

@@ -21,6 +21,6 @@ stdenv.mkDerivation rec {
     homepage = "http://goodies.xfce.org/projects/panel-plugins/${p_name}";
     description = "Mailwatch plugin for Xfce panel";
     platforms = platforms.linux;
-    maintainers = [ maintainers.matthiasbeyer ];
+    maintainers = [ ];
   };
 }

pkgs/desktops/xfce/panel-plugins/xfce4-mpc-plugin.nix

@@ -21,6 +21,6 @@ stdenv.mkDerivation rec {
     homepage = "http://goodies.xfce.org/projects/panel-plugins/${p_name}";
     description = "MPD plugin for Xfce panel";
     platforms = platforms.linux;
-    maintainers = [ maintainers.matthiasbeyer ];
+    maintainers = [ ];
   };
 }

pkgs/desktops/xfce/panel-plugins/xfce4-timer-plugin.nix

@@ -25,6 +25,6 @@ stdenv.mkDerivation rec {
     description = "Battery plugin for Xfce panel";
     platforms = platforms.linux;
     license = licenses.gpl2;
-    maintainers = [ maintainers.matthiasbeyer ];
+    maintainers = [ ];
   };
 }