player/audio: remove explicit drain on EOF

We have previously had a problem where pull AOs (such as pipewire)
would not reinitialize on a format change when going between two
audio-only files. In such a situation, playback would stop after the
first file.

We initially attempted to fix this by explicitly draining on EOF, which
solves that problem but introduces a blocking step where we don't
actually want one, breaking gapless audio, and causing dropped frames
at the end of playback for files with video in them too.

So, let's undo these changes and do something better in the next
commit.

player/audio.c

@@ -945,8 +945,6 @@ void fill_audio_out_buffers(struct MPContext *mpctx)
                           (opts->gapless_audio && !ao_untimed(ao_c->ao))))
         {
             MP_VERBOSE(mpctx, "audio EOF reached\n");
-            if (mpctx->ao)
-                ao_drain(mpctx->ao);
             mpctx->audio_status = STATUS_EOF;
             mp_wakeup_core(mpctx);
         }