MediaPlayerService: Remove unnecessary zero writes to AudioSink

Causes glitches in PCM playback for NuPlayer callback mode.

Not sure the original intention of this with AwesomePlayer, as zeroes
are only placed under EOS condition for AudioSink targets, not AudioTrack
targets.  Does not seem useful anymore as AwesomePlayer appears to
call stop() which should send the remaining track data out.

Bug: 21198655
Change-Id: Iab80dd7bc6e2e341f92406c5d8f8085e9d877bcd