panel_simple_remove() returns zero unconditionally. Make it return no value
instead making more obvious what happens in the callers.

This is a preparation for making platform and mipi-dsi remove callbacks
return void.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220708094922.1408248-2-u.kleine-koenig@pengutronix.de