Remove BandaidTransferListener.

It has now become an empty shell as the real TransferListener provides all its
methods.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=203950443

library/core/src/main/java/com/google/android/exoplayer2/upstream/BandwidthMeter.java

@@ -47,5 +47,6 @@ interface EventListener {
    * Returns the {@link TransferListener} that this instance uses to gather bandwidth information
    * from data transfers. May be null, if no transfer listener is used.
    */
-  @Nullable TransferListener<?> getTransferListener();
+  @Nullable
+  TransferListener<? super DataSource> getTransferListener();
 }

library/core/src/main/java/com/google/android/exoplayer2/upstream/DefaultBandwidthMeter.java

@@ -172,7 +172,7 @@ public synchronized long getBitrateEstimate() {
   }
 
   @Override
-  public @Nullable TransferListener<?> getTransferListener() {
+  public @Nullable TransferListener<? super DataSource> getTransferListener() {
     return this;
   }