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/button/button.cpp

@@ -13,8 +13,5 @@ void Button::press() {
 }
 void Button::add_on_press_callback(std::function<void()> &&callback) { this->press_callback_.add(std::move(callback)); }
 
-void Button::set_device_class(const std::string &device_class) { this->device_class_ = device_class; }
-std::string Button::get_device_class() { return this->device_class_; }
-
 }  // namespace button
 }  // namespace esphome