fix yieldThread memory leak

[syusui-s]

I noticed that the current implementation of yieldThread will consume memory.
The message channel will never be used after the call but there is still a 'message' call-back function so GC cannot free it.

It is needed to call removeEventHandler.

Detail

If you run this script with node command like node script.js, the process will never finish.

async function yieldThread() {
  return new Promise(resolve => {
    const ch = new MessageChannel()
    ch.port1.addEventListener('message', resolve)
    ch.port2.postMessage(0)
    ch.port1.start()
  })
}

yieldThread()

If there is removeEventListener, it will finish successfully.

async function yieldThread() {
  return new Promise(resolve => {
    const ch = new MessageChannel()
    const handler = () => {
      ch.port1.removeEventListener('message', handler)
      resolve()
    }
    ch.port1.addEventListener('message', handler)
    ch.port2.postMessage(0)
    ch.port1.start()
  })
}

yieldThread();