Remove single-version-externally-managed in setup.cfg
Remove single-version-externally-managed in setup.cfg

The single-version-externally-managed flag is no longer
a config option in pbr, it is now the default behaviour
in normal setuptools.

Change-Id: I5173d78680afca534b28cefa155636d9d695b1d9

moshele/old-networking-mlnx