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>

clutter/clutter/clutter-input-device.c

@@ -52,8 +52,6 @@ enum
 {
   PROP_0,
 
-  PROP_BACKEND,
-
   PROP_NAME,
 
   PROP_DEVICE_TYPE,
@@ -89,8 +87,6 @@ struct _ClutterInputDevicePrivate
 
   ClutterSeat *seat;
 
-  ClutterBackend *backend;
-
   char *vendor_id;
   char *product_id;
   char *node_path;
@@ -198,10 +194,6 @@ clutter_input_device_set_property (GObject      *gobject,
       priv->device_mode = g_value_get_enum (value);
       break;
 
-    case PROP_BACKEND:
-      priv->backend = g_value_get_object (value);
-      break;
-
     case PROP_NAME:
       priv->device_name = g_value_dup_string (value);
       break;
@@ -272,10 +264,6 @@ clutter_input_device_get_property (GObject    *gobject,
       g_value_set_enum (value, priv->device_mode);
       break;
 
-    case PROP_BACKEND:
-      g_value_set_object (value, priv->backend);
-      break;
-
     case PROP_NAME:
       g_value_set_string (value, priv->device_name);
       break;
@@ -400,18 +388,6 @@ clutter_input_device_class_init (ClutterInputDeviceClass *klass)
                           FALSE,
                           CLUTTER_PARAM_READWRITE | G_PARAM_CONSTRUCT_ONLY);
 
-  /**
-   * ClutterInputDevice:backend:
-   *
-   * The #ClutterBackend that created the device.
-   */
-  obj_props[PROP_BACKEND] =
-    g_param_spec_object ("backend",
-                         P_("Backend"),
-                         P_("The backend instance"),
-                         CLUTTER_TYPE_BACKEND,
-                         CLUTTER_PARAM_READWRITE | G_PARAM_CONSTRUCT_ONLY);
-
   /**
    * ClutterInputDevice:vendor-id:
    *

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,

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,