Removing default setting (#2716)

`discovery: true` is on by default and therefore not required here.  You only need this to set `discovery: false`

components/mqtt.rst

@@ -149,7 +149,6 @@ discovery in your Home Assistant configuration with the following:
     # Example Home Assistant configuration.yaml entry
     mqtt:
       broker: ...
-      discovery: true
 
 And that should already be it 🎉 All devices defined through ESPHome should show up automatically
 in the entities section of Home Assistant.
@@ -308,7 +307,6 @@ Also make sure to change the ``port`` of the mqtt broker. Most brokers use port
     mqtt:
       broker: test.mymqtt.local
       port: 8883
-      discovery: true
       discovery_prefix: ${mqtt_prefix}/homeassistant
       log_topic: ${mqtt_prefix}/logs
       # Evaluate carefully skip_cert_cn_check
@@ -353,7 +351,6 @@ MQTT can have some overrides for specific options.
     name: "Component Name"
     # Optional variables:
     retain: true
-    discovery: true
     availability:
       topic: livingroom/status
       payload_available: online