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
Merge "Remove single-version-externally-managed in setup.cfg"
Imported Translations from Transifex

Change-Id: I49837fb8f8771c532c2fd09d67598ad839a2f7a3
Imported Translations from Transifex

Change-Id: I9b95e89cfaf76537d98ada38fb795fbaf439c996