There is no need to support non-pip users…

We don’t need that file.  pip uninstalls before running setup.py install.
Let’s not support non-pip (and non-$SANE_PACKAGE_MANAGER) users.

Revert "add an install_plugin.py to override old things"

This reverts commit 0c49e7be7faa7fed2b9f77e8c896347aef07c07f.