xprtrdma: Remove rr_func

A posted rpcrdma_rep never has rr_func set to anything but
rpcrdma_reply_handler.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Tested-By: Devesh Sharma <devesh.sharma@avagotech.com>
Reviewed-by: Doug Ledford <dledford@redhat.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>