Remove argparse from requirements

argparse was external in python 2.6 but not anymore, remove it from
requirements.

This should help with pip 8.0 that gets confused in this situation.
Installation of the external argparse is not needed.

Change-Id: Ib7e74912b36c1b5ccb514e31fac35efeff57378d
diff --git a/requirements.txt b/requirements.txt
index 01fd245..8388f0b 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,6 +1,5 @@
 pbr>=1.1.0
 
-argparse
 PyYAML>=3.1.0
 Paste
 WebOb>=1.2.3