bus-proxy: never apply policy when sending signals

Unlike dbus-daemon, the bus-proxy does not know the receiver of a
broadcast (as the kernel has exclusive access on the bus connections).
Hence, and "destination=" matches in dbus1 policies cannot be applied.

But kdbus does not place any restrictions on *SENDING* broadcasts, anyway.
The kernel never returns EPERM to KDBUS_CMD_SEND if KDBUS_MSG_SIGNAL is
set. Instead, receiver policies are checked. Hence, stop checking sender
policies for signals in bus-proxy and leave it up to the kernel.

This fixes some network-manager bus-proxy issues where NM uses weird
dst-based matches against interface-based matches. As we cannot perform
dst-based matches, our bus-proxy cannot properly implement this policy.