drm/tests: helpers: Drop empty platform remove function

A remove callback just returning 0 is equivalent to no remove callback
at all. So drop the useless function.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20230224221417.1712368-1-u.kleine-koenig@pengutronix.de