avformat/mpegtsenc: fix flushing of audio packets

avformat/mpegtsenc: fix flushing of audio packets

7d097a0fc57f0fa8385962a539c657c2f40b5ed0 had the same purpose as
3700f655c55e2001b57215210b957b169d66b50f but the former is much simpler, so
let's remove the latter.

Unfortunately both checks were wrong, because in order to make sure DTS > PCR
we have to give us some headroom, so instead of using a dts_difference <
max_delay check let's use a dts_difference < max_delay/2 check.

Fixes DTS < PCR errors with this command line:

./ffmpeg -loglevel verbose -y -f lavfi -i \
  "testsrc=s=64x64:d=20,split=2[out0][tmp1];[tmp1]vflip[out1];sine=d=20,asetnsamples=1000[out2]" \
  -flags +bitexact -fflags +bitexact -sws_flags +accurate_rnd+bitexact  \
  -codec:v libx264 -codec:a mp2 -b:a 32k -pix_fmt yuv420p \
  -map '0:v:0' \
  -map '0:v:1' \
  -map '0:a:0'  \
  -muxrate 800000 \
  -program st=0:st=2 -program st=1:st=2 -program st=2 -program st=0 -f mpegts out1.ts

Signed-off-by: Marton Balint <cus@passwd.hu>