How Can I drop system.users collection

[Filip Havlíček]

How Can I drop system.users collection in database? In previsus versions of MongoDB every database has its own system.users collection but in newer (I think since 2.6) are credentials stored in admin.system.users. I try to drop obsolete collection (somedatabase.system.users), but I can't.

rs:PRIMARY> db.system.users.drop()
2015-09-21T13:55:09.502+0200 drop failed: {
    "ok" : 0,
    "errmsg" : "not authorized on somedatabase to execute command { drop: \"system.users\" }",
    "code" : 13
} at src/mongo/shell/collection.js:598

mongo 2.6.10
user rights:
{
    "_id" : "admin.user",
    "user" : "user",
    "db" : "admin",
    "roles" : [
        {
            "role" : "readWriteAnyDatabase",
            "db" : "admin"
        },
        {
            "role" : "userAdminAnyDatabase",
            "db" : "admin"
        },
        {
            "role" : "dbAdminAnyDatabase",
            "db" : "admin"
        },
        {
            "role" : "clusterAdmin",
            "db" : "admin"
        }
    ]
}