pybind/mgr: remove vestigial basepython directives

pybind/mgr: remove vestigial basepython directives

Remove what I think are vestigial configuration parameters setting
`basepython` in tox.ini. These may have been useful during the change
from python 2 to 3, but I don't see them serving any useful purpose
at this current time. In fact they now interfere with the ability to
globally override the basepython version of all testenv's at once.
So let's just remove them. We can always add it back if we find
an issue in the future (and document it!).

Signed-off-by: John Mulligan <jmulligan@redhat.com>