(streams/delete-from-index can also take a predicate.

Merges delete-by's behavior into delete-from index. (delete-from-index
index [:host :state]) is a stream which deletes matching events from the
index.
Remove streams/delete-from-index and streams/update-index.

Nobody is using multiple indexes, and having 3 copies of these functions
was getting confusing. riemann.core deletes and updates a core's index,
and riemann.config proxies to it. Configuration files will be unaffected
by this change, unless they explicitly referenced
streams/delete-from-index or streams/update-index.