I couldn't find a png to edit for it in the gui folder, and it's not set to anything in the file. If you just want the fastest way to hide it, you can just open the MainMenuWindow_online.json file and change the very first
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 "RenderColor": "255,255,255,255",
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to
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 "RenderColor": "0,255,255,255",
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The buttons are children of the box so if you just hide it, you'll hide the buttons too. If you need to remove the Main Menu title that prints, you can hide it separately. Kind of a lame solution, but it's easy and it works.
Triangle's post in How do I get this off my screen? was marked as the answer