sunrpc: Remove unused xprt_iter_get_xprt

xprt_iter_get_xprt() was added by
commit 80b14d5e ("SUNRPC: Add a structure to track multiple
transports") but is unused.

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Acked-by: Anna Schumaker <anna.schumaker@oracle.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>