After converting master/slave replication to replica set (as described on https://docs.mongodb.org/manual/core/master-slave/#convert-a-master-slave-deployment-to-a-replica-set), I cannot drop local.oplog.$main collection:

Error: drop failed:

{ "ok" : 0, "errmsg" : "can't drop collection with reserved $ character in name" }

I need to drop it since
1. it has allocated around 4 GB of space
2. it is not needed (local.oplog.rs used instead)
Renaming an oplog is not possible.

Could you please advise on how to drop it?

Thank you.