ALSA: line6: Remove unused line6_midibuf_skip_message()

Use of this function ended with commits 3e58c868db ("staging: line6:
drop midi_mask_receive") and af89d2897a ("staging: line6: drop
midi_mask_transmit".)

[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>

sound/usb/line6/midibuf.h

@@ -29,8 +29,6 @@ extern int line6_midibuf_init(struct midi_buffer *mb, int size, int split);
 extern int line6_midibuf_read(struct midi_buffer *mb, unsigned char *data,
 			      int length);
 extern void line6_midibuf_reset(struct midi_buffer *mb);
-extern int line6_midibuf_skip_message(struct midi_buffer *mb,
-				      unsigned short mask);
 extern int line6_midibuf_write(struct midi_buffer *mb, unsigned char *data,
 			       int length);