mtd: move mxcnd_remove to .exit.text

The function mxcnd_remove is used only wrapped by __exit_p so define it
using __exit.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Vladimir Barinov <vova.barinov@gmail.com>
Cc: Vladimir Barinov <vbarinov@embeddedalley.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>