code: function removeUser(db, username, options, callback) {
    let Db = loadDb();
  
    // Attempt to execute command
    executeAuthRemoveUserCommand(db, username, options, (err, result) => {
      if (err && err.code === -5000) {
        const finalOptions = applyWriteConcern(Object.assign({}, options), { db }, options);
        // If we have another db set
        const db = options.dbName ? new Db(options.dbName, db.s.topology, db.s.options) : db;
  
        // Fetch a user collection
        const collection = db.collection(CONSTANTS.SYSTEM_USER_COLLECTION);
  
        // Locate the user
        findOne(collection, { user: username }, finalOptions, (err, user) => {
          if (user == null) return handleCallback(callback, err, false);
          remove(collection, { user: username }, finalOptions, err => {
            handleCallback(callback, err, true);
          });
        });
  
        return;
      }
  
      if (err) return handleCallback(callback, err);
      handleCallback(callback, err, result);
    });
  }
docs: Remove a user from a database
  
  @method
  @param {Db} db The Db instance on which to remove the user.
  @param {string} username The username.
  @param {object} [options] Optional settings. See Db.prototype.removeUser for a list of options.
  @param {Db~resultCallback} [callback] The command result callback