ALSA: seq: Drop dead code for the old broadcast support

The broadcast and multicast supports have been never enabled.
Let's drop the dead code.

Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20230523075358.9672-21-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>