haskellPackages: reflect removal of pkgs.webkitgtk_4_0

hackage2nix can't run if there are unrecoverable errors while generating
the nixpkgs package Nix. To create this change, I temporarily added
webkitgtk_4_0 = null to all-packages.nix to solve those errors. Then
running ./maintainers/scripts/haskell/regenerate-hackage-packages.sh --fast
effects the desired change.

See also NixOS/cabal2nix#674.

Author: sternenseemann