...for now. Eventually, we will want a signal that tells us when sources
are no longer used, however that isn't just limited to when they're
deleted! This new signal should also be dispatched, for example, when
a source image field is set to `None`. Since none of the built-in
strategies are currently using the source_deleted signal, I've decided
to remove it until we have a more complete solution.
As discussed in #214, source_created and source_changed didn't really
have clear definitions. In truth, their names and separation betray
their origins as model receivers in earlier versions. The "source group"
abstraction helped us get away from thinking about things exclusively in
terms of models, but these remained as an artifact.
