microdvd: do not export framerate hint as subtitle packet

MicroDVD has a "hack" for specifying the video framerate the subtitle
was authored against. The demuxer reads this hint correctly, but didn't
skip it correctly.

This was not noticed, because the exported packet has its duration set
to 0, making it invisible (depending on the API user's rendering logic).

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
