Remove persist/--policy-dir option

This is the first in series of Spring cleaning around the storage layer.
In the near future we will add local disk-based persistence support to
OPA. That support will handle storage of source files.

The --policy-dir option is almost entirely unused today. Removing it
will make it easier to get rid of the policyStore entirely.

The next thing to do will be to remove the specialized *Policy methods
from the storage layer. This way the storage layer can just accept
policies as normal data.

If policies need to be persisted until then, users can treat the
policies as config files and manage them outside of OPA.