Make implicit ACKs work on MQTT

[GUVWAF]

Implicit ACKs when sending a message to a channel didn’t work when using purely MQTT, because we were filtering out messages we originally sent.

I think it’s safe to remove this, because we do check for the gateway_id already in order to avoid loops with MQTT:

firmware/src/mqtt/MQTT.cpp

Lines 133 to 134 in 16a3a32

	if (strcmp(e.gateway_id, owner.id) == 0)
	LOG_INFO("Ignoring downlink message we originally sent.\n");

Just checked with two devices connected to MQTT with LoRa Tx disabled and I do get ACKs now on the primary channel. The device will create an ACK when receiving the rebroadcast, so it also avoids unnecessary retransmissions.

ACKs for DMs work already.