Currently as part of the drop database coordinator flow, we broadcast _flushDatabaseCacheUpdates  command with majority write concern to all shards and we wait synchronously for its execution.
The only purpose of this call is to flush the cached entries of the dropped database from all the nodes in the cluster to reclaim memory. So it is not strictly required.
For this reason I think we should:
 - stop using the majority write concern
 - use fireAndForget to asynchronously schedule the command execution on all the shards and not block this coordinator.