doc: remove ffmpeg-mt-authorship.txt from master (for upcoming 0.11)

This lists git history, which is better kept in the history itself
that is both ours as this file as well as the actual history of the
ffmpeg-mt project.
If someone thinks this is not ok, drop me a mail and ill put it back!

Also note this file was not carried in our previous release with noone
complaining
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>