fltk: don't build PDF docs
fltk: don't build PDF docs

- it was broken, probably after doxygen update:
  https://hydra.nixos.org/build/171898654
- HTML is more than enough nowadays, I believe
- the texlive-medium build-dep was relatively expensive

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