clutter/input-device: Remove backend property

It will conflict with a MetaInputDevice property that'll have the same
name.

Wasn't set by the native backend anyway, so probably harmless.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2718>

src/backends/x11/meta-seat-x11.c

@@ -651,7 +651,6 @@ create_device (MetaSeatX11    *seat_x11,
                          "device-type", source,
                          "capabilities", capabilities,
                          "device-mode", mode,
-                         "backend", clutter_backend,
                          "vendor-id", vendor_id,
                          "product-id", product_id,
                          "device-node", node_path,