1134972 - remove calls to mongo flush

During orphan deletion, calls to flush the mongo db were being made after some
commits. This is not something Pulp should be doing, mongo should be able to
handle this on its own. Additionally, the flush calls required special DBA
roles on the pulp user in mongo.

The flush calls during orphan removal have been removed, which allows pulp to
operate without special mongo roles.
Merge pull request #1144 from beav/no-flush

1134972 - remove calls to mongo flush