README: Remove "Shell Integration" section

This is better encapsulated as a recommendation in, say, `direnv`'s readme.
Our repo has arguably too much documentation already
and this is only somewhat related to our domain.

We may want to re-instantiate this as a "TIPS.md" or something,
but I don't feel that this belongs in the top-level rendered README.

README.md

@@ -258,41 +258,6 @@ nix_direnv_watch_file mod1.nix mod2.nix
 use flake
 ```
 
-## Shell integration
-
-To quickly add a `default.nix`/`flake.nix` to a project you can put the following snippets in your `.bashrc`/`.zshrc`:
-
-```bash
-nixify() {
-  if [ ! -e ./.envrc ]; then
-    echo "use nix" > .envrc
-    direnv allow
-  fi
-  if [[ ! -e shell.nix ]] && [[ ! -e default.nix ]]; then
-    cat > default.nix <<'EOF'
-with import <nixpkgs> {};
-mkShell {
-  nativeBuildInputs = [
-    bashInteractive
-  ];
-}
-EOF
-    ${EDITOR:-vim} default.nix
-  fi
-}
-
-flakify() {
-  if [ ! -e flake.nix ]; then
-    nix flake new -t github:nix-community/nix-direnv .
-  elif [ ! -e .envrc ]; then
-    echo "use flake" > .envrc
-    direnv allow
-  fi
-  ${EDITOR:-vim} flake.nix
-}
-
-```
-
 ## Known Bugs
 
 At the moment `nix-direnv` depends on GNU Grep and a modern Bash version.