Remove USB support

Hi I want to remove the USB stuff on the Zero in order to use the pins as GPIO.
can I just remove the build.usb_flags in plattform.txt or do I need to remove other definitions on other files?

there are no Zero pins used by the USB


on pin 33 and 34 USB is connected.
I edited the pin definitions in variant.cpp and sucessfully used those pins however to be sure that no false switching on these pins occur I'd like to remove the USB control etc

-DUSBCON from build.usb_flags in platform.txt is used over boards.txt in main.cpp