Remove useless code after USB transfer

Found by Rian Hunter (rian@alum.mit.edu), patch via discord:

This code was incorrect in the case where the bytes written during tud_audio_write() was more than the side of the endpoint. That case actually never happens because we always write less than USB_AUDIO_PAYLOAD_SIZE and USB_AUDIO_PAYLOAD_SIZE is actually the size of the endpoint.

Still, the code as it was has no real effect and it's error-prone in case certain transfer parameters change.