xprtrdma: Simplify rpcrdma_deregister_external() synopsis

Clean up: All remaining callers of rpcrdma_deregister_external()
pass NULL as the last argument, so remove that argument.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Tested-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>