xprtrdma: No qp_event disconnect

After thinking about this more, and auditing other kernel ULP imple-
mentations, I believe that a DISCONNECT cm_event will occur after a
fatal QP event. If that's the case, there's no need for an explicit
disconnect in the QP event handler.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>