Remove unneeded null check

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=208208099

library/hls/src/main/java/com/google/android/exoplayer2/source/hls/HlsMediaSource.java

@@ -437,9 +437,7 @@ public final class HlsMediaSource extends BaseMediaSource
 
 
   @Override
   @Override
   public void releaseSourceInternal() {
   public void releaseSourceInternal() {
-    if (playlistTracker != null) {
-      playlistTracker.stop();
-    }
+    playlistTracker.stop();
   }
   }
 
 
   @Override
   @Override