Issue #28440: No longer add /Library/Python/site-packages, the Apple-supplied

Issue #28440: No longer add /Library/Python/site-packages, the Apple-supplied
system Python site-packages directory, to sys.path for macOS framework builds.
The coupling between the two Python instances often caused confusion and, as
of macOS 10.12, changes to the site-packages layout can cause pip component
installations to fail.  This change reverts the effects introduced in 2.7.0
by Issue #4865.  If you are using a package with both the Apple system Python
2.7 and a user-installed Python 2.7, you will need to ensure that copies of
the package are installed with both Python instances.