Drop default SIDs

[dburgener]

Currently if a policy doesn't specify any initial_context() calls, there are default types (kernel_sid, security_sid and unlabled_sid) generated for the mandatory sids, and all the sid configuration is done. This could potentially be opt-in behavior, but doing it implicitly isn't the right answer, since the user won't even know about these types to work with them. The blocker on removing them now is that the tests rely on this behavior. So we'll need to augment the tests to add a file of initial_context() calls in all tests in order to avoid breaking nearly all the tests while removing the default initial_contexts()