Scheduler: Add `unschedule` method

Jobs registered with `schedule()` can't be removed at runtime. This is needed for bots that conditionally disable scheduled tasks.

### Proposed API
```python
def unschedule(self, name: str) -> None
```

### Example
```python
bot.scheduler.unschedule("morning_update")
```

### Before opening a PR
- Write unit tests
- Run `mypy matrix/`
- Run `pytest tests/`
- Run `black .`