vapoursynth: remove flags for optional plugins
vapoursynth: remove flags for optional plugins

Since R55 vapoursynth no longer bundles any plugins.

This also creates an empty directory at $out/lib/vapoursynth, which now
no longer exists, to suppress a warning about this directory missing.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs