These scripts rely on some Python library path manipulation to be
able to load from the source tree.  But, in fact that are a few
reasons for not having those, including:

 * All functionality in `avocado` relies on plugins, which will not be
   available from the source tree unless a `$ setup.py develop` is
   executed, which automatically makes the "import magic" unnecessary.

 * Most users will end up using a setuptools generated "entry point"
   instead.

For the sake of less code, I propose this (and all other?) magic to be
removed.
Signed-off-by: NCleber Rosa <crosa@redhat.com>