Remove deprecated MediaSource.prepareSource.

This method only ensured backward compatibility while the new version was not
public yet. Now ExoPlayer 2.9.0. is public, this workaround can be removed.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=217881691

library/core/src/main/java/com/google/android/exoplayer2/source/BaseMediaSource.java

@@ -135,12 +135,6 @@ public abstract class BaseMediaSource implements MediaSource {
 
   @Override
   public final void prepareSource(
-      ExoPlayer player, boolean isTopLevelSource, SourceInfoRefreshListener listener) {
-    prepareSource(player, isTopLevelSource, listener, /* mediaTransferListener= */ null);
-  }
-
-  @Override
-  public final void prepareSource(
       ExoPlayer player,
       boolean isTopLevelSource,
       SourceInfoRefreshListener listener,

library/core/src/main/java/com/google/android/exoplayer2/source/MediaSource.java

@@ -220,11 +220,6 @@ public interface MediaSource {
    */
   void removeEventListener(MediaSourceEventListener eventListener);
 
-  /** @deprecated Will be removed in the next release. */
-  @Deprecated
-  void prepareSource(
-      ExoPlayer player, boolean isTopLevelSource, SourceInfoRefreshListener listener);
-
   /**
    * Starts source preparation if not yet started, and adds a listener for timeline and/or manifest
    * updates.