drm/sun4i: tv: Remove useless function

The drm_connector_to_sun4i_tv() function isn't used anywhere in the driver,
so let's remove it.

Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220728-rpi-analog-tv-properties-v2-37-459522d653a7@cerno.tech