mptcp: drop useless addr_signal clear

msk->pm.addr_signal is cleared in mptcp_pm_add_addr_signal, no need to
clear it in mptcp_pm_nl_add_addr_send_ack again. Drop it.

Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>