rgb nightlight outlet: remove white mode.

Despite appearing in the portal, it does not seem possible to put this into a real white mode.  The light is RGB, not RGBW, and has no seperate brightness for white light.

Issue #360

custom_components/tuya_local/devices/rgb_nightlight_outlet.yaml

@@ -17,8 +17,6 @@ secondary_entities:
         type: string
         name: color_mode
         mapping:
-          - dps_val: white
-            value: white
           - dps_val: colour
             value: rgbw
           - dps_val: scene