Move DeleteSlide logic to `Remove` adapter action

[Tom-Kingstone]

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<int> slideNumbers = removeRequest.SlideNumbers;

    // Delete slide logic here...

    return slideNumbers.Count;
}