Don't hard-code PYTHONPATH for brew test

This rules out any possibility to test Python 3.x software as the
PYTHONPATH is used by any Python and `brew test pyqt --with-python3`
fails because it picks up the `pyqt` module fomr Python 2.x's
site-packages. Upstream tells us not to set the PYTHONPATH and then
starting another (major) python version.

Library/Homebrew/formula.rb

@@ -508,8 +508,6 @@ class Formula
   def test
     require 'test/unit/assertions'
     extend(Test::Unit::Assertions)
-    # TODO: move this into PythonInstalled.
-    ENV['PYTHONPATH'] = PythonInstalled.new.global_site_packages
     ret = nil
     mktemp do
       @testpath = Pathname.pwd