Remove all files but the pin_mapping.json from filesystem on factory reset

This allows to create more user defined files and get them deleted on factory reset.

include/Utils.h

@@ -11,4 +11,5 @@ public:
     static int getTimezoneOffset();
     static void restartDtu();
     static bool checkJsonAlloc(const DynamicJsonDocument& doc, const char* function, const uint16_t line);
+    static void removeAllFiles();
 };