Remove legacy unique_id field from entities (#9022)

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

esphome/components/text_sensor/text_sensor.cpp

@@ -70,7 +70,5 @@ void TextSensor::internal_send_state_to_frontend(const std::string &state) {
   this->callback_.call(state);
 }
 
-std::string TextSensor::unique_id() { return ""; }
-
 }  // namespace text_sensor
 }  // namespace esphome