gst-ptp-helper: Do not disable multicast loopback

Otherwise we cannot run gst-ptp-helper if the PTP master is on the
same device.

Part-of: <gstreamer/gstreamer!1546>
39 jobs for !1546 with backport-mr-1306-into-1.18-enable-multicast-loopback-ptp-helper in 51 minutes and 54 seconds (queued for 30 seconds)
merge request