Error logged if orphaned shipping method remains after disabling module

Certainly not a huge issue, but the following error is logged on every request if the Timeslots module is disabled when a Timeslots shipping method still exists:
```
Could not load class: TimeSlotsShippingMethod from mysql.timeslotsshippingmethod.
```

The Timeslots shipping method won't be viewable to delete on the Commerce configuration screen, so the module needs to be re-enabled first.

