[ALSA] USB midi: Remove duplicate CS_AUDIO_* #defines

Removed the CS_AUDIO_* #defines, which were duplicates of the
class-specific USB_DT_CS_* #defines in <linux/usb_ch9.h>.

Signed-off-by: Ben Williamson <ben.williamson@greyinnovation.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>