Remove inconsistent override in CDropdown.h

It was added in 670f1e5d42 and, while not illegal, was inconsistent with the
rest of the header file, which caused compiler warnings.
As a quick solution the override keyword is simply removed again.
In the long run, it would be good to still modernise the style of all files
in that directory (at once).

source/gui/ObjectTypes/CDropDown.h

@@ -74,7 +74,7 @@ public:
 	 */
 	virtual void Draw(CCanvas2D& canvas);
 
-	virtual void CreateJSObject() override;
+	virtual void CreateJSObject();
 
 	// This is one of the few classes we actually need to redefine this function
 	//  this is because the size of the control changes whether it is open