Mendix apps are designed for responsiveness.
The hamburger meu is (the only item) displayed when the screen size reduces and you no longer see the available navigation items. It will help the user to toggle the menu and open the available navigation option. You can easily check this by reducing your browser screen and if for instance you use top bar navigation then the options are hidden for smaller screen sizes.
I’d suggest you to reconsider removing them entirely. Maybe add some css to display it only for screen sizes below a certain value, you can use media queries to do that.
Hope this helps!
Check the layout of your pages. In the web layout you can just delete that icon if not needed.
Also, you can have have separate layouts for mobile and web.
You should bare in mind that in Mendix you can have different layouts for different devices so firstly make sure you know which layouts your app is using.
I hope this helps a bit on your way to knowing where to find these layouts and their settings. Here you can mainly changed everything about them, but keep in mind that all pages which use these layouts will reflect all changes you make here because they are the core layouts so it is not advised to just go changing/deleting everything here.