Remove version string from the setup.cfg

PBR handles setting the version based on the git tags, so we don't need
to do this in the setup.cfg anymore.

Change-Id: I3c12f54682b58c9eea5b40fa2c34979775c0b827
diff --git a/setup.cfg b/setup.cfg
index 1e7cc2b..2de9f34 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -1,6 +1,5 @@
 [metadata]
 name = tempest
-version = 4
 summary = OpenStack Integration Testing
 description-file =
     README.rst