Reduce Username in MQTT Config to account for PKI overhead

[garthvh]

When adding the PKI functionality it appears that we have not accounted for the extra overhead and have taken MQTT config over the packet size limit.

The map report config needs to be removed and we need to reduce the size of the username in order to get back under the packet size limit.

No config section should be larger than can be remotely administered.

[caveman99]

for reference: current #define meshtastic_ModuleConfig_MQTTConfig_size 254