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/tests/meta-backend-test.c

@@ -161,7 +161,6 @@ meta_backend_test_add_test_device (MetaBackendTest        *backend_test,
                          "device-type", CLUTTER_TOUCHSCREEN_DEVICE,
                          "seat", seat,
                          "has-cursor", has_cursor,
-                         "backend", clutter_backend,
                          "vendor-id", "MetaTest",
                          "product-id", product_id,
                          "n-buttons", n_buttons,