drm/sun4i: tcon: remove unused function

Even though that function is defined in the TCON header, it's not defined
nor used anywhere. Remove the prototype.
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>

drivers/gpu/drm/sun4i/sun4i_tcon.h

@@ -194,8 +194,6 @@ void sun4i_tcon_channel_enable(struct sun4i_tcon *tcon, int channel);
 void sun4i_tcon_enable_vblank(struct sun4i_tcon *tcon, bool enable);
 
 /* Mode Related Controls */
-void sun4i_tcon_switch_interlace(struct sun4i_tcon *tcon,
-				 bool enable);
 void sun4i_tcon_set_mux(struct sun4i_tcon *tcon, int channel,
 			struct drm_encoder *encoder);
 void sun4i_tcon0_mode_set(struct sun4i_tcon *tcon,