cmake: remove obsolete substituteInPlace (#113843)

This `substituteInPlace` was introduced in 2016 when cmake was at v3.4:

* https://github.com/NixOS/nixpkgs/commit/5286c0aebb

In 2018 cmake v3.12.0 changed the code to make it not match anything:

* https://github.com/Kitware/CMake/commit/50b5e9ed13

```
substituteStream(): WARNING: pattern '${PYTHON_EXECUTABLE}' doesn't match anything in file 'Modules/FindCxxTest.cmake'
```

Having it around in 2021 with cmake v3.19.3 doesn't help,
and tests are disabled anyway.

pkgs/development/tools/build-managers/cmake/default.nix

@@ -64,8 +64,6 @@ stdenv.mkDerivation (rec {
       --subst-var-by libc_bin ${lib.getBin stdenv.cc.libc} \
       --subst-var-by libc_dev ${lib.getDev stdenv.cc.libc} \
       --subst-var-by libc_lib ${lib.getLib stdenv.cc.libc}
-    substituteInPlace Modules/FindCxxTest.cmake \
-      --replace "$""{PYTHON_EXECUTABLE}" ${stdenv.shell}
   ''
   # CC_FOR_BUILD and CXX_FOR_BUILD are used to bootstrap cmake
   + ''