openvswitch: remove python-six dependency

openvswitch: remove python-six dependency

Python six was required to build the OVS Python libs during the time when
they were supporting both Python 2 & 3.

Python 3 is a minimum requirement for OVS Python's libs since commits:
  https://github.com/openvswitch/ovs/commit/1ca0323e7c29dc7ef5a615c265df0460208f92de
  https://github.com/openvswitch/ovs/commit/bd9052455092630e03485538a29f9f8d147c41ca
and Six is no longer required since commit
  https://github.com/openvswitch/ovs/commit/0c4d144a989a444d038d58272d8571e97e00e86f

The end-goal here is to get rid of the Python Six host-build.
OVS is the only user.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>