ALSA: Remove the last mention of SNDRV_MAIN_OBJECT_FILE

SNDRV_MAIN_OBJECT_FILE hasn't done anything since the pre-git days, and
the only remaining reference occurs as a #define in sound/last.c.  Drop
that last mention of it.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>