[BOARDS] Remove Arduino boards (#1929)

docs/use/gateway.md

@@ -77,10 +77,6 @@ By default this function is not available on the pre built binary of RFBridge, i
 ```
 build_flags = '-UMQTTsetMQTT'
 ``` 
-Arduino boards does not have this function per default also, to add it:
-```
-build_flags = '-DMQTTsetMQTT'
-``` 
 :::
 
 ::: tip