ALSA: pcm: Drop unused snd_pcm_substream.file field

It's assigned but nowhere used.  Let's remove it.
Signed-off-by: Takashi Iwai <tiwai@suse.de>

include/sound/pcm.h

@@ -470,7 +470,6 @@ struct snd_pcm_substream {
 	struct snd_pcm_group self_group;	/* fake group for non linked substream (with substream lock inside) */
 	struct snd_pcm_group *group;		/* pointer to current group */
 	/* -- assigned files -- */
-	void *file;
 	int ref_count;
 	atomic_t mmap_count;
 	unsigned int f_flags;

sound/core/oss/pcm_oss.c

@@ -2427,7 +2427,6 @@ static int snd_pcm_oss_open_file(struct file *file,
 		}
 
 		pcm_oss_file->streams[idx] = substream;
-		substream->file = pcm_oss_file;
 		snd_pcm_oss_init_substream(substream, &setup[idx], minor);
 	}
 	

sound/core/pcm_native.c

@@ -2452,10 +2452,8 @@ static int snd_pcm_open_file(struct file *file,
 		return -ENOMEM;
 	}
 	pcm_file->substream = substream;
-	if (substream->ref_count == 1) {
-		substream->file = pcm_file;
+	if (substream->ref_count == 1)
 		substream->pcm_release = pcm_release_private;
-	}
 	file->private_data = pcm_file;
 
 	return 0;