Remove dead code.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=118954433

demo/src/main/java/com/google/android/exoplayer/demo/PlayerActivity.java

@@ -287,7 +287,6 @@ public class PlayerActivity extends Activity implements SurfaceHolder.Callback,
   // Internal methods
 
   private SourceBuilder getSourceBuilder() {
-    String userAgent = Util.getUserAgent(this, "ExoPlayerDemo");
     switch (contentType) {
       case Util.TYPE_SS:
         return new SmoothStreamingSourceBuilder(dataSourceFactory, contentUri.toString(),