fix: respect DontMqttMeBro flag regardless of channel PSK (#9626)

The previous PSK check was broken from its introduction in #4643 —
memcmp was used in boolean context without comparing to 0, inverting
the condition. Since no one noticed for over a year, the PSK-based
filtering provided no practical value. Simplifying to always respect
the sender's preference is both more correct and easier to reason about.

src/mqtt/MQTT.cpp

@@ -754,10 +754,8 @@ void MQTT::onSend(const meshtastic_MeshPacket &mp_encrypted, const meshtastic_Me
     if (mp_decoded.which_payload_variant == meshtastic_MeshPacket_decoded_tag) {
         // For uplinking other's packets, check if it's not OK to MQTT or if it's an older packet without the bitfield
         bool dontUplink = !mp_decoded.decoded.has_bitfield || !(mp_decoded.decoded.bitfield & BITFIELD_OK_TO_MQTT_MASK);
-        // check for the lowest bit of the data bitfield set false, and the use of one of the default keys.
-        if (!isFromUs(&mp_decoded) && !isMqttServerAddressPrivate && dontUplink &&
-            (ch.settings.psk.size < 2 || (ch.settings.psk.size == 16 && memcmp(ch.settings.psk.bytes, defaultpsk, 16)) ||
-             (ch.settings.psk.size == 32 && memcmp(ch.settings.psk.bytes, eventpsk, 32)))) {
+        // Respect the DontMqttMeBro flag for other nodes' packets on public MQTT servers
+        if (!isFromUs(&mp_decoded) && !isMqttServerAddressPrivate && dontUplink) {
             LOG_INFO("MQTT onSend - Not forwarding packet due to DontMqttMeBro flag");
             return;
         }