MediaPlayer2: clean up libstagefright_player2

- Remove unnecessary files in the source list.
- Remove ClearDataSourceFactory no longer used.

Background: The removed files are no longer used thanks to the recent
refactoring of GenericSource2.

Test: build
Bug: 109928575
Change-Id: I5a27779513e0ed6189c6d3627d455b4e99a19aaa