Bluetooth: Make mgmt_index_added() and mgmt_index_removed() return void

The return value from mgmt_index_added() and mgmt_index_removed()
functions is never used. So do not pretend that returning an error
would actually be handled and just make both functions return void.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>