sourcegraph: remove GRPC index methods (#977)
This was part of an effort to move the queue from Zoekt to Sourcegraph.
However, we are not going to pursue this for now and we can remove
the corresponging grpc methods.
Sourcegraph never had callers of Index and Delete, so both are save to remove.
`DeleteAllData` is still being called from Sourcegraph.
Test plan:
CI