The Reply in thread feature does an implicit user mention

### Steps to reproduce

1. Start in a room view which contains some threaded messages
2. Long-tap on a threaded message
3. Tap "Reply in thread"
4. Type and send a message

### Outcome

#### What did you expect?

A regular message posted to the thread (`m.relates_to` being populated with the thread / event information).

I don't expect that the message would be mentioning anyone explicitly. I merely wish to post a new threaded message.

#### What happened instead?

The newly created message event is posted under the thread correctly (`m.relates_to` is OK), but it also **mentions the person you "replied to"** (`m.mentions` is populated).

This affects my usage of [baibot](https://github.com/etkecc/baibot), as doing a mention of the bot (as Element X iOS does right now) invokes different behavior than what would have happened when merely continuing the thread and not mentioning anyone.

### Your phone model

iPhone 14

### Operating system version

iOS 18.1

### Application version

Version 1.9.6 (137)

### Homeserver

Synapse 1.119.0

### Will you send logs?

No