Removes JSON constructor from UI elements

This fixes the error: clang:optin.cplusplus.VirtualCall
When virtual methods are called during construction and destruction the polymorphism is restricted to the class that’s being constructed or destructed because the more derived contexts are either not yet initialized or already destructed.

UI/ui_led.hpp

@@ -10,7 +10,6 @@ class UI_led : public UI_item
   public:
     UI_led(QObject *parent = nullptr);
     UI_led(std::string text, Color fg_color, Color bg_color, QObject *parent = nullptr);
-    UI_led(const json &j, QObject *parent = nullptr);
 
     bool set_led(bool state, Color on, Color off);