func (db *Database) RemoveUser(user string) error {
	err := db.Run(bson.D{{"dropUser", user}}, nil)
	if isNoCmd(err) {
		users := db.C("system.users")
		return users.Remove(bson.M{"user": user})
	}
	if isNotFound(err) {
		return ErrNotFound
	}
	return err
}
func (db *Database) RemoveUser(user string) error {
	err := db.Run(bson.D{{"dropUser", user}}, nil)
	if isNoCmd(err) {
		users := db.C("system.users")
		return users.Remove(bson.M{"user": user})
	}
	if isNotFound(err) {
		return ErrNotFound
	}
	return err
}
// RemoveUser removes the authentication credentials of user from the database.