The WebM DASH Manifest muxer can write manifests for live streams and
these contain an entry that depends on the time the manifest is written;
an AVOption to make the output reproducible has been added for tests.
But this is unnecessary, as there already is a method for reproducible
output: The AVFMT_FLAG_BITEXACT-flag of the AVFormatContext. Therefore
this commit removes the custom option.

Given that the description of said option contained "private option -
users should never set this" and that it was not documented in
muxers.texi, no deprecation period for this option seemed necessary.

The commands of the FATE-tests for this muxer have been changed to no
longer use this option.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>