fix: remove `Misconduct::on_signal`

Essentially we may want a interface with the ability to:
	i) keep as severity window (2 * era_len) -> slash after that time has expired
	ii) slash `on_end_of_era`

Then `on_signal` becomes useless.

However, it would require a change on `Slashing::slash` which probably should return `Option<MisconductLevel>`

That would return `Some(MisconductLevel)` when a window has expired otherwise return `None`.
A `reporter` would need to call `slash` even if misconducts had occured