debian: Drop the dependency on software-properties-gtk

This will let GNOME Software use its own software repository management
dialog instead of software-properties-gtk.

Fixes #3.

debian/control

@@ -55,7 +55,6 @@ Depends: appstream,
          gsettings-desktop-schemas (>= 3.18),
          libgtk3-perl,
          packagekit (>= 1.1.11),
-         software-properties-gtk,
          ${misc:Depends},
          ${shlibs:Depends}
 Conflicts: sessioninstaller

debian/control.in

@@ -51,7 +51,6 @@ Depends: appstream,
          gsettings-desktop-schemas (>= 3.18),
          libgtk3-perl,
          packagekit (>= 1.1.11),
-         software-properties-gtk,
          ${misc:Depends},
          ${shlibs:Depends}
 Conflicts: sessioninstaller