xprtrdma: Remove opcode check in Receive completion handler

Clean up: The opcode check is no longer necessary, because since
commit 2fa8f88d ("xprtrdma: Use new CQ API for RPC-over-RDMA
client send CQs"), this completion handler is invoked only for
RECV work requests.

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