[ALSA] kill dead code

ALSA<-OSS emulation
The Coverity checker found this obviously dead code.

I'm not sure which of the if (plugin == NULL) is correct - this patch
removes the one that couldn't be true.
Signed-off-by: NAdrian Bunk <bunk@stusta.de>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>

sound/core/oss/pcm_plugin.c

@@ -663,10 +663,7 @@ static int snd_pcm_plug_playback_channels_mask(snd_pcm_plug_t *plug,
 		bitset_t *dstmask = bs;
 		int err;
 		bitset_one(dstmask, schannels);
-		if (plugin == NULL) {
-			bitset_and(client_vmask, dstmask, schannels);
-			return 0;
-		}
+
 		while (1) {
 			err = plugin->src_channels_mask(plugin, dstmask, &srcmask);
 			if (err < 0)