android.media.MediaMuxer: remove SAMPLE_SYNC_FLAG

SAMPLE_SYNC_FLAG is not used by MediaMuxer; instead,
MediaCodec.BUFFER_FLAG_SYNC_FRAME is used, which has
the same value.

Remove this now, so that users will not have to translate
MediaCodec flags to MediaMuxer flags, even though MediaMuxer
takes in MediaCodec.BufferInfo objects to specify these flags.

Change-Id: I4b2f2039ca16debf4788a530a36bdd06d516f417
Signed-off-by: Lajos Molnar <lajos@google.com>
Bug: 9169479