mailbox: arm_mhuv2: make remove callback return void

My build tests failed to catch that amba driver that would have needed
adaption in commit 3fd269e7 ("amba: Make the remove callback return
void"). Change the remove function to make the driver build again.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 3fd269e7 ("amba: Make the remove callback return void")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>