Remove debug specific handling

I think debug handling was initially done this way for CLI handling
where we wanted to make sure only the correct information was printed to
the console. However as logging.basicConfig sets up a stream handler on
the root logging object I can't see any purpose to the debug handling in
the actual HTTPClient. Further than this it is completely wrong that a
client library is messing with it's logging level, this should be handled
by an application.

The debug flag is maintained and deprecated in HTTPClient and removed
from the session object. There has been no release since the addition of
session so there is no problem with compatibility.

Change-Id: Ib00f3d93d099ed1a9dd25f17121610a7289f0061