ALSA: line6: Remove unused line6_midibuf_status()

This function has not been used since merging the driver into the kernel
(and a good while before that.)

[Removed the corresponding line in midibuf.h, too -- tiwai]

Signed-off-by: Chris Rorvick <chris@rorvick.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>