python3.pkgs.pybind11: clean redundant "out = " argument builtins.derivation

Nixcpp allows passing argument to builtins.derivation that conflict with
output names, but it silently ignores them. For example,

	derivation {
	  name = "test";
	  system = builtins.currentSystem;
	  builder = "/bin/sh";
	  args = ["-c" ": > $out"];
	  out = "banana";
	}

evaluates to the same derivation no matter the explicit value of "out".
Any coercible value produces the same result.

pkgs/development/python-modules/pybind11/default.nix

@@ -21,7 +21,6 @@ let
   setupHook = makeSetupHook {
     name = "pybind11-setup-hook";
     substitutions = {
-      out = placeholder "out";
       pythonInterpreter = python.pythonOnBuildForHost.interpreter;
       pythonIncludeDir = "${python}/include/${python.libPrefix}";
       pythonSitePackages = "${python}/${python.sitePackages}";