SDK / exoplayer

Remove self-links, and remove section that is documenting internal
ordering behaviour of
[`SimpleBasePlayer.getCombinedMediaMetadata`](https://github.com/google/ExoPlayer/blob/bb270c62cf2f7a1570fe22f87bb348a2d5e94dcf/library/common/src/main/java/com/google/android/exoplayer2/SimpleBasePlayer.java#L1770)
rather than anything specifically about this method.

#minor-release

PiperOrigin-RevId: 490923719
(cherry picked from commit a6703285)