On Wayland, fix nightly warnings

The new analysis suggests that we can remove mut.

src/platform_impl/linux/wayland/seat/keyboard/mod.rs

@@ -166,7 +166,7 @@ impl KeyboardHandler for WinitState {
         modifiers: Modifiers,
     ) {
         let modifiers = ModifiersState::from(modifiers);
-        let mut seat_state = self.seats.get_mut(&keyboard.seat().id()).unwrap();
+        let seat_state = self.seats.get_mut(&keyboard.seat().id()).unwrap();
         seat_state.modifiers = modifiers;
 
         // NOTE: part of the workaround from `fn enter`, see it above.

src/platform_impl/linux/wayland/seat/touch/mod.rs

@@ -103,7 +103,7 @@ impl TouchHandler for WinitState {
         let seat_state = self.seats.get_mut(&touch.seat().id()).unwrap();
 
         // Remove the touch point.
-        let mut touch_point = match seat_state.touch_map.get_mut(&id) {
+        let touch_point = match seat_state.touch_map.get_mut(&id) {
             Some(touch_point) => touch_point,
             None => return,
         };