CLEANUP: connection: use conn_xprt_ready() instead of checking the flag

CLEANUP: connection: use conn_xprt_ready() instead of checking the flag

It's easier and safer to rely on conn_xprt_ready() everywhere than to
check the flag itself. It will also simplify adding extra checks later
if needed. Some useless controls for !xprt have been removed, as the
XPRT_READY flag itself guarantees xprt is set.