Remove the use of resource autodeletion in unit tests

The Neutron unit tests currently decorate resource creation methods with contextlib.contextmanager to allow resources to be automatically deleted when they are no longer needed. This strategy is a legacy of a time when this explicit cleanup was required to ensure isolation between tests, but is now unnecessary since db state is automatically thrown away after each test. Given that autodeletion has an execution and code maintenance penalty, its removal is adviseable.