your syntax looks good to me

benydc.js

#!/usr/bin/env node
'use strict';

const mongoose = require('mongoose');
mongoose.connect('mongodb://localhost/test');
const conn = mongoose.connection;
const Schema = mongoose.Schema;

const schema = new Schema({
  events: [Schema.Types.ObjectId]
});

const Test = mongoose.model('test', schema);

const test = new Test({ 
  events: [
    mongoose.Types.ObjectId(),
    mongoose.Types.ObjectId(),
    mongoose.Types.ObjectId()
  ]
});

const toRemove = test.events[2];

console.log(`removing: ${toRemove}`);

async function run() {
  await conn.dropDatabase();
  await test.save();
  await Test.findByIdAndUpdate(test._id, { $pull: { events: toRemove } });
  let found = await Test.findOne();
  console.log(found);
  return conn.close();
}

run();

output.txt

$: ./benydc.js
removing: 5b195ff3cbf271283f9cbcbf
{ events: [ 5b195ff3cbf271283f9cbcbd, 5b195ff3cbf271283f9cbcbe ],
  _id: 5b195ff3cbf271283f9cbcc0,
  __v: 0 }

$: