net/smc: remove Kconfig warning

Now separate memory regions are created and registered for separate
RMBs. The unsafe_global_rkey of the protection domain is no longer
used. Thus the exposing memory warning can be removed.

Signed-off-by: Ursula Braun <ubraun@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>