python{2,3}Packages.bootstrapped-pip: remove extraneous entry from PYTHONPATH

From my testing, wheel is only retrieved from its installation in $out,
and removing it doesn't break anything. I'm unsure if it was required when
bootstrapping was initially added [0], but it definitely isn't now.

I also took the chance to clarify the comment a bit.

[0]: 56727dc1ff

pkgs/development/python2-modules/bootstrapped-pip/default.nix

@@ -38,9 +38,10 @@ stdenv.mkDerivation rec {
     mv pip* pip
     mv setuptools* setuptools
     mv wheel* wheel
-    # Set up PYTHONPATH. The above folders need to be on PYTHONPATH
-    # $out is where we are installing to and takes precedence
-    export PYTHONPATH="$out/${python.sitePackages}:$(pwd)/pip/src:$(pwd)/setuptools:$(pwd)/setuptools/pkg_resources:$(pwd)/wheel:$PYTHONPATH"
+    # Set up PYTHONPATH:
+    # - pip and setuptools need to be in PYTHONPATH to install setuptools, wheel, and pip.
+    # - $out is where we are installing to and takes precedence, and is where wheel will end so we can install pip.
+    export PYTHONPATH="$out/${python.sitePackages}:$(pwd)/pip/src:$(pwd)/setuptools:$(pwd)/setuptools/pkg_resources:$PYTHONPATH"
 
     echo "Building setuptools wheel..."
     pushd setuptools