Email notifications show broken images (due to authenticated media)

### Description

Reported on behalf of a conversation I saw: Email notifications use the legacy, unauthenticated, and deprecated URLs for avatars of rooms/senders, which don't work when authenticated media is in full force. This leads to the "broken image" placeholder in browsers/email clients.

The easiest fix is to remove the `img` elements entirely, as it's questionable to include them from a Trust & Safety perspective anyways (at least until there's better tooling available). Magic URLs like how bridges work may be desirable in the long term.

### Steps to reproduce

* receive an email notification

### Homeserver

matrix.org

### Synapse Version

whatever matrix.org runs

### Installation Method

Other (please mention below)

### Database

postgresql

### Workers

Single process

### Platform

matrix.org

### Configuration

_No response_

### Relevant log output

```shell
see matrix.org
```

### Anything else that would be useful to know?

_No response_