Remove orphaned flavors

Currently, when a resource class is deleted, the
associated tuskar flavors are not deleted.  This is
problematic, especially because ids are re-used,
creating unintended associations between new
resource classes and old flavors.

This patch removes the tuskar flavors from the database.

Partial-Bug: #1233267
Change-Id: I3d0046ba927faf095e3a66ccb538270c78745228