Use reverse index to remove multihash index content

[gammazero]

Now that reverse indexing is available, it could be used to remove all of the multihashes that no longer map to any metadata. This could be used instead of the current GC process which needs to re-read ads for a removed context. The ability to cleanup multihashes associated with a removed context could all be done in RemoveProviderContext.

Alternatively, the reverse index could be used by GC as another method of acquiring the multihashes associated with a removed context.