python36: remove symlink to pip

Symbolic links were added pointing to the executables that end with 3 as
part of the Python 2 to 3 migration. At some point I disabled ensurepip
but forgot to remove this symbolic link.

pkgs/development/interpreters/python/cpython/3.6/default.nix

@@ -125,7 +125,6 @@ in stdenv.mkDerivation {
 
     # Use Python3 as default python
     ln -s "$out/bin/idle3" "$out/bin/idle"
-    ln -s "$out/bin/pip3" "$out/bin/pip"
     ln -s "$out/bin/pydoc3" "$out/bin/pydoc"
     ln -s "$out/bin/python3" "$out/bin/python"
     ln -s "$out/bin/python3-config" "$out/bin/python-config"