Remove legacy unique_id field from entities (#9022)

Co-authored-by: Jesse Hills <3060199+jesserockz@users.noreply.github.com>

esphome/components/adc/adc_sensor_esp8266.cpp

@@ -56,8 +56,6 @@ float ADCSensor::sample() {
   return aggr.aggregate() / 1024.0f;
 }
 
-std::string ADCSensor::unique_id() { return get_mac_address() + "-adc"; }
-
 }  // namespace adc
 }  // namespace esphome