It has been included since af5f434f
for deprecation reasons, but removing it has been forgotten after
it had served is purpose. So remove it.

For convenience, include version.h instead as LIBAVUTIL_VERSION_INT
is supposed to be used when creating AVClasses.
Reviewed-by: Martin Storsjö <martin@martin.st>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>