After I updated the firmware a new button appeared in my AV Sources, which is Mirror AV. I have no use for this button as I don't have another screen and it's a huge waste of space. I've messed around in the menus and I can't seem to find an option to remove it.

nope, its part of the system now. gotta live with it.

...thus continuing Pioneer's tradition of not consulting with a single human interface or GUI design expert before releasing software updates.

In this case, any undergraduate human factors engineering major in college would've told them not to waste screen real estate (and add confusion) by putting up a button that 99% of the users of the product will NEVER touch. The same college student would've told them not to change the icon for the iPod source button to THE CONNECTOR for the iPod instead of the actual iPod icon itself that used to be there. Just idiotic. What's next....changing the icons for the AM and FM radio sources to a little image of, say, a capacitor?