ptp-helper: Allow sync to master clock on same host

If we drop all messages with the same clock id as ours we will also drop all messages coming from a PTP clock on our host since both clock ids are build from the same MAC address.

At least for Linux we do not see our own messages anyway since the network stack can well distinguish between multicast send from our socket or from another socket on the same machine. To make sure that this works for all supported platforms just drop delay requests since this is the only message that is sent from the GStreamer PTP clock.