bash-completion: remove NixOS-specific patch

The new bash-completion support in NixOS doesn't require this patch anymore.
Besides, the patch was insufficient for most purposes anyway: Bash completion
modules are spread out over all user profiles listed in $NIX_PROFILES (plus the
current-system profile), so getting full support for all installed modules
requires support for more than one "bash_completion.d" directory anyway.

pkgs/shells/bash-completion/default.nix

@@ -11,11 +11,6 @@ stdenv.mkDerivation {
     sha256 = "e5a490a4301dfb228361bdca2ffca597958e47dd6056005ef9393a5852af5804";
   };
 
-  postInstall = ''
-    sed -i "$out/share/bash-completion/bash_completion" \
-        -e 's|: .{BASH_COMPLETION_COMPAT_DIR:=.*}|BASH_COMPLETION_COMPAT_DIR="$HOME/.nix-profile/etc/bash_completion.d"|'
-  '';
-
   doCheck = true;
 
   meta = {