Account deactivation doesn't work on Conduit servers

### Steps to reproduce

1. Sign in to account on a Conduit homeserver
1. Go to Settings
1. Tap Deactivate Account button. Doesn't seem to matter whether or not the forget my messages option is selected.
1. Enter your password (error happens whether or not you enter correct password)

### Outcome

#### What happened instead?
`M_BAD_JSON`

### Your phone model

_No response_

### Operating system version

_No response_

### Application version

_No response_

### Homeserver

Conduit 0.3.0

### Will you send logs?

No

Reproduce account deactivation against a Conduit 0.3.0 homeserver using both password outcomes and both message options, then trace the iOS deactivation flow that produces M_BAD_JSON. Done means the request is handled correctly and account deactivation succeeds or presents the server error clearly.

Written by the indexing model from the issue text.