vf: remove most GPL video filters

Almost all of them had their guts removed and replaced by libavfilter
long ago, but remove them anyway. They're pointless and have been
scheduled for deprecation.

Still leave vf_format (because we need it in some form) and vf_sub (not
sure).

This will break some builtin functionality: lavfi yadif defaults are
different, auto rotation and stereo3d downconversion are broken. These
might be fixed later.