Keep Device Class in Flash. (#4639)

* Keep Device Class in Flash.

* Remove blank line

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Co-authored-by: Your Name <you@example.com>

esphome/components/binary_sensor/binary_sensor.cpp

@@ -43,12 +43,7 @@ void BinarySensor::send_state_internal(bool state, bool is_initial) {
 }
 
 BinarySensor::BinarySensor() : state(false) {}
-void BinarySensor::set_device_class(const std::string &device_class) { this->device_class_ = device_class; }
-std::string BinarySensor::get_device_class() {
-  if (this->device_class_.has_value())
-    return *this->device_class_;
-  return "";
-}
+
 void BinarySensor::add_filter(Filter *filter) {
   filter->parent_ = this;
   if (this->filter_list_ == nullptr) {