ALSA: line6: Remove unused line6_midibuf_skip_message()

Use of this function ended with commits 3e58c868 ("staging: line6:
drop midi_mask_receive") and af89d289 ("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>