pre-commit/check-merge-conflicts-2: fix use outside dev shell

Note that this is just a script that is meant to run outside a
derivation (but also can be called by a derivation builder).
`touch $out` does not belong in it.

`touch $out` worked accidentally in the derivation-based check,
and also in the dev shell, but if pre-commit is invoked without
the dev shell it would fail.

maintainers/flake-module.nix

@@ -35,7 +35,6 @@
                 echo "ERROR: found merge/patch conflicts in files"
                 exit 1
               fi
-              touch $out
             ''}";
           };
           clang-format = {