Remove unsupported redaction reasons

### Description

Redaction reasons are currently not supported in order to avoid abuse #17241, matrix-org/matrix-react-sdk#5604.
The fact that Element still offers the input field is confusing and, in my eyes, a defect.
Especially, the reason is still saved and shown when viewing hidden events.

Add the feature to the backlog and move it behind a labs flag for the time being.

Logs being sent: no

### Version information

- **Platform**: desktop

For the desktop app:

- **OS**: Arch
- **Version**: aur/element-desktop-nightly-bin 2021051101-1