MediaPlayer2: remove unused code setMetadataFilter and getMetadata

1. These are dead codes since there is not entry points in the
   MediaPlayer2[Impl].java side.
2. In MP1, setMetadataFilter() and getMetadata() has been hidden
   from the beginning. Ic20240a0b388897950c98337b539eb05e487273a
   And, even the Metadata class is deprecated in favor of MediaMetadata.

Test: build
Bug: 112767225

Change-Id: Ia76ca34faf45ca03d04b8599e312e87ea4a3486b