Don't double destruct audio_track_cblk_t

Fortunately audio_track_cblk_t doesn't have a destructor, but for clarity
remove the double destruction.

Also add warning not to add any virtuals to audio_track_cblk_t.

Change-Id: I70ebe1a70460c7002145b2cdf10f9f137396e6f3
diff --git a/include/private/media/AudioTrackShared.h b/include/private/media/AudioTrackShared.h
index dd97ce4..23226c0 100644
--- a/include/private/media/AudioTrackShared.h
+++ b/include/private/media/AudioTrackShared.h
@@ -54,6 +54,7 @@
 #define CBLK_RESTORED_ON        0x0040  // track has been restored after invalidation
 #define CBLK_RESTORED_OFF       0x0040  // by AudioFlinger
 
+// Important: do not add any virtual methods, including ~
 struct audio_track_cblk_t
 {