ALSA: Drop leftover snd-rtctimer stuff from Makefile

We forgot to remove the line for snd-rtctimer from Makefile while
dropping the functionality.  Get rid of the stale line.

Fixes: 34ce71a9

 ("ALSA: timer: remove legacy rtctimer")
Link: https://lore.kernel.org/r/20240221092156.28695-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>