Deletion of Bopomofo IME does not work when typing on iPhone / 打字的時候無法刪除注音

### Is this a hackmd.io issue?

- [x] Yes, the bug happens on https://hackmd.io
- [ ] No. If it's a self-hosted CodiMD instance, please fire up the issue in the repo of CodiMD: https://github.com/hackmdio/codimd

### Read our Official tutorial

It's on https://hackmd.io/c/tutorials/

或是中文版 https://hackmd.io/c/tutorials-tw/

### What's the problem

Deletion of Bopomofo IME will delete the chars in HackMD only, not the chars in IME

#### Current behaviour

Provide steps to reproduce:

1. Use build-in Traditional Chinese Bopomofo IME
2. There are already some text in the note
3. type something in IME and tap **delete** button

Please also refer to this video to reproduce:
https://github.com/dwy6626/issue-artifacts/blob/master/HackMD/HackMD-Bopomofo-iPhone.MP4

#### Expected behavior

Should delete chars in IME instead

#### Environment

I use HackMD on:

**Mobile**

- Device: iPhone 8
- OS: iOS 12.3.1
- Browser: Chrome 76.0.3809.81