Add remove method to StorageBackendInterface
Add remove method to StorageBackendInterface

Define a remove method for removing/deleting items from the storage
backend. The tuf reference implementation needs to be able to delete
metadata files which have been marked as removed.

Signed-off-by: Joshua Lock <edc229f50a11a11cdece0270af6320fb299dca1e@vmware.com>

secure-systems-lab/securesystemslib,secure-systems-lab/securesystemslib