Delete transitional method EncodedImage.maybeRetain

Bug: webrtc:9378
Change-Id: Ibe3d5bad835d1725faa38f8e2a804efc9272776e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/155661
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30072}

sdk/android/api/org/webrtc/RefCounted.java

@@ -18,7 +18,7 @@ package org.webrtc;
  */
 public interface RefCounted {
   /** Increases ref count by one. */
-  void retain();
+  @CalledByNative void retain();
 
   /**
    * Decreases ref count by one. When the ref count reaches zero, resources related to the object