setup: undo (for the second time) the use of the --multi-version feature

setup: undo (for the second time) the use of the --multi-version feature
When this feature is turned on, then setuptools doesn't create easy-install.pth, setuptools.pth, or site.py in the target site-packages dir.  I don't know why not and we should probably open a ticket on the setuptools tracker and/or hack setuptools to create those files anyway.  But for now (for the Tahoe-1.3.0 release), we're going to leave --multi-version mode off and require users to manually uninstall any packages which are too old and thus conflict with our newer dependencies.