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

sound/core/Makefile

@@ -32,7 +32,6 @@ snd-ump-objs      := ump.o
 snd-ump-$(CONFIG_SND_UMP_LEGACY_RAWMIDI) += ump_convert.o
 snd-timer-objs    := timer.o
 snd-hrtimer-objs  := hrtimer.o
-snd-rtctimer-objs := rtctimer.o
 snd-hwdep-objs    := hwdep.o
 snd-seq-device-objs := seq_device.o