Drop metrics_dashboard_annotations table to support cells 1.0 sharding

The metrics_dashboard_annotations table is awaiting a sharding key. However, Metrics::Dashboard::Annotations model does not exist anymore (as the feature was in the process of being removed during a reorg).

Because of this, we won't be able to backfill sharding key data based on the associated model and it's relationships.

We should drop the metrics_dashboard_annotations table.