OS X tries to turn on every mic connector so that Siri will work. I use 2 Apple Cinema displays connected to my 2013 Mac Pro. I've left 1 camera enabled for Skype with tape over it, but I've unplugged the microphone connector. If I WANT a microphone, I'll plug in my headset.
The glass simply pulls off as it's held by magnets.
Remove 10 screws holding the LCD and carefully lift it up to expose the circuit board below. Unplug as you see fit.