Drop RPC securemessage.py and crypto module.

As of 579109cab946a54f86b04724f0b6cb71fb027c04 Nova should
have the openstack common crypto modules to support
rpc/securemessage.

The RPC securemessage stuff will eventually get integrated into
oslo.message. As is it is only partially complete in oslo-incubator
and it is getting synced into various projects (and breaking
some things).

This commit drops securemessage.py which was missing classes in
the crypto module anyway and would not have worked.

Closes-bug #1241634

Change-Id: I050d620a51042007aa4696fa439bd99c7597a9a4
Merge "Drop RPC securemessage.py and crypto module."