qtmux: Only write timecode trak for video

Recent changes in ccextractor were attaching timecode meta to the closed
caption track. We shouldn't write timecode information for the closed
caption trak.
8 jobs for <span class="ref-name">qtmux_no_tc_for_cc</span> in 19 minutes and 25 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #104445
gst indent

00:00:30

passed #104444
manifest

00:00:36

 
  Build
passed #104446
build fedora x86_64

00:03:09

 
  Test
passed #104447
check fedora

00:03:36

 
  Full Builds
passed #104448
build android arm64 api28

00:05:20

passed #104450
build cerbero cross win32

00:12:02

passed #104451
build cerbero cross win64

00:06:39

passed #104449
build cerbero fedora x86_64

00:10:27