Refactor MQTT::onReceive to reduce if/else nesting (#5592)

* Refactor MQTT::onReceive to reduce if/else nesting

* Fix missing #include <functional>

* const DecodedServiceEnvelope e

* Combine validDecode if statement.

* Only call pb_release when validDecode.

* s/ptr/channelName/

* Use reference type for deleter

* Use lambda instead of bind

* Document deleter

* Reorder 'if's to avoid object creation

* Remove unnecessary comment

* Remove 'else'; simpifies #5516

---------

Co-authored-by: Ben Meadors <benmmeadors@gmail.com>

src/mqtt/MQTT.h

@@ -117,9 +117,6 @@ class MQTT : private concurrency::OSThread
     // Check if we should report unencrypted information about our node for consumption by a map
     void perhapsReportToMap();
 
-    // returns true if this is a valid JSON envelope which we accept on downlink
-    bool isValidJsonEnvelope(JSONObject &json);
-
     /// Determines if the given address is a private IPv4 address, i.e. not routable on the public internet.
     /// These are the ranges: 127.0.0.1, 10.0.0.0-10.255.255.255, 172.16.0.0-172.31.255.255, 192.168.0.0-192.168.255.255.
     bool isPrivateIpAddress(const char address[]);