Move DeleteSlide logic to `Remove` adapter action

#### Broken rules:

Deleting slides should be handled in the `Remove` action, instead of `Push`. This will also resolve some confusion after #16 is completed, where a user could create a slide before a removed slide gets removed.

#### Suggestions to restore compliance:

Create an IRequest that contains a list of integers that will correspond to slide numbers, and handle deleting slides with that request in `Remove` instead of in `Push`

something along the lines of:

```
public override int Remove(IRequest request, ActionConfig actionConfig = null)
{
RemoveSlidesRequest removeRequest = request as RemoveSlidesRequest;

List slideNumbers = removeRequest.SlideNumbers;

// Delete slide logic here...

return slideNumbers.Count;
}
```