Automatically set mqtt buffer size (#637)

* Automatically set mqtt buffer size

Current versions of pubsubclient allows the buffer size to be set
dynamically: https://github.com/knolleary/pubsubclient/pull/282

We can take advantage of this to avoid having to modify one of the
dependencies and instead automatically up the buffer size when a
powerful board is detected.

platformio.ini

@@ -118,7 +118,6 @@ lib_deps =
   ${libraries.wifimanager}
 build_flags =
   ${env.build_flags}
-  -DMQTT_MAX_PACKET_SIZE=1024
   '-DsimpleReceiving=true'
   '-DZmqttDiscovery="HADiscovery"'
   '-DTRACE=1'
@@ -130,7 +129,6 @@ lib_deps =
   ${libraries.ethernet}
 build_flags =
   ${env.build_flags}
-  -DMQTT_MAX_PACKET_SIZE=1024
   '-DsimpleReceiving=true'
   '-DZmqttDiscovery="HADiscovery"'
   '-DTRACE=1'
@@ -141,7 +139,6 @@ lib_deps =
   ${libraries.ethernet}
 build_flags =
   ${env.build_flags}
-  -DMQTT_MAX_PACKET_SIZE=128
 
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