Remove a superfluous check of sock->fd against -1

Remove a superfluous check of sock->fd against -1

The check is left from when tcp_connect_direct() called isc__nm_socket()
and it was uncertain whether it had succeeded, but now isc__nm_socket()
is called before tcp_connect_direct(), so sock->fd cannot be -1.

    *** CID 357292:    (REVERSE_NEGATIVE)
    /lib/isc/netmgr/tcp.c: 309 in isc_nm_tcpconnect()
    303
    304     	atomic_store(&sock->active, true);
    305
    306     	result = tcp_connect_direct(sock, req);
    307     	if (result != ISC_R_SUCCESS) {
    308     		atomic_store(&sock->active, false);
    >>>     CID 357292:    (REVERSE_NEGATIVE)
    >>>     You might be using variable "sock->fd" before verifying that it is >= 0.
    309     		if (sock->fd != (uv_os_sock_t)(-1)) {
    310     			isc__nm_tcp_close(sock);
    311     		}
    312     		isc__nm_connectcb(sock, req, result, true);
    313     	}
    314