update-flake.sh: remove support for Nix < 2.19

Nix 2.24 is the default in NixOS 24.11.

This is required by the following commit that uses a `flake update`
command syntax only supported by Nix >= 2.19.

helper/update-flake.sh

@@ -39,22 +39,8 @@ if [[ $forceRun ]] && ! git diff --quiet ../flake.{nix,lock}; then
     exit 1
 fi
 
-# Support Nix >=2.19
-{
-    versionGreaterThanOrEqual() {
-        [[ $1 != $(echo -e "$1\n$2" | sort -V | head -n1) || $1 == "$2" ]]
-    }
-    nixVersion=$(nix --version | cut -d\  -f 3)
-    if versionGreaterThanOrEqual "$nixVersion" 2.19; then
-        # https://nixos.org/manual/nix/stable/release-notes/rl-2.19#:~:text=nix%20flake%20update
-        nixUpdateArg=--flake
-    else
-        nixUpdateArg=
-    fi
-}
-
 echo "Updating flake 'nixos-search'"
-nix flake update $nixUpdateArg ../test/nixos-search
+nix flake update --flake ../test/nixos-search
 echo
 
 versions=$(nix eval --json -f update-flake.nix versions)
@@ -66,7 +52,7 @@ echo "Updating main flake"
 if [[ $nixosVersion ]]; then
     sed -Ei "s|(nixpkgs.url = .*nixos-)[^\"]+|\1$nixosVersion|" ../flake.nix
 fi
-nix flake update $nixUpdateArg ..
+nix flake update --flake ..
 
 echo
 nix eval --raw -f update-flake.nix --argstr prevVersions "$versions" showUpdates; echo