make birectional channels the default, clean up some channel handling

lib/serial/GwSerial.h

@@ -42,7 +42,6 @@ class GwSerial : public GwChannelInterface{
         virtual Stream *getStream(bool partialWrites);
         bool getAvailableWrite(){return availableWrite;}
         virtual void begin(unsigned long baud, uint32_t config=SERIAL_8N1, int8_t rxPin=-1, int8_t txPin=-1)=0;
-        virtual String getMode() override;
         virtual int getType() override;
     friend GwSerialStream;
 };