The packages argument was incorrectly specified, leading to an install
process that would appear to have succeeded, but would not actually copy
code.

The error likely occurred due to copying setup.py from another project
where the source files were located in a different position relative to
setup.py.

Also remove usused imports and outdated doc text.

Now, to reference mininet files, use 'import mininet.mininet'.

PYTHONPATH mods are no longer required for installation.