Drop neutron.common.rpc.MessagingTimeout

Drop the last neutron.common.rpc exception mapping, MessagingHelper,
which wasn't actually used anywhere anyway.

Part of blueprint drop-rpc-compat.

Change-Id: I43b9b97accff9cc7684146076ba0b5d62302f06e
enable F811 check for flake8

This change incorporates two cleanups that do not change logic:
- Removes the shadowed unused imports by using the proper oslo.config import
mechanism
- duplicate unit tests have been removed
- duplicate unit test names have been corrected to reflect true test
  nature

Change-Id: Iad7a3373c1f787a2078b932c5f40bb718ad76767
Partial-Bug: 1329017
Merge "Drop neutron.common.rpc.MessagingTimeout"