see here:
https://www.pjrc.com/teensy/td_midi.html

when connected to a host that is sending midi, it is important to clear the input buffer. otherwise the host device/software is liable to stall or do other bad stuff.

(er, i should say that i have not actually tested this myself, going by 2nd-hand reports.)

oh right: also commented out the serial port initialization, since its unused; (and in the context of norns means the host is trying to figure out what to do with a non-functional usb-serial port.)