Revert "Add pymysql to requirements.txt"

This reverts commit fa09c747c1deb2f67070808da8c7ec2431091a93.

requirements.txt

@@ -25,6 +25,5 @@ python-swiftclient>=3.2
 pywinrm
 PyYAML
 requests
-pymysql
 sqlalchemy
 webob