TEST_MAPPING: remove comment for android.media.audio.cts.AudioPlaybackCaptureTest

This comment was for an already-closed ticket. AudioPlaybackCaptureTest
was not even excluded anymore. I still tested it and it passed 50/50 on
an internal-main x86_64 Cuttlefish device.

Bug: 149314419
Test: atest
CtsMediaAudioTestCases: android.media.audio.cts.AudioPlaybackCaptureTest
--rerun-until-failure 50
Change-Id: Icad12de0d981529369d312dda049ce6aa4bc6769
Signed-off-by: Lucas Henneman <henneman@google.com>