ALSA: hda/generic: Drop obsoleted obey_preferred_dacs flag

Now we evaluate directly with preferred_dacs table, the flag is no
longer used and merely a placeholder.
Let's drop the definition and its users.

Link: https://patch.msgid.link/20241001121439.26060-2-tiwai@suse.de


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