Remove superseded SelectionMover class

The SelectionMover class has been superseded by more flexible combinations of filters, scanners and utils. The replacements bring less dependence on the cursor node for positioning, allowing greater flexibility and re-use.

Removing this code and associated tests performs the final step necessary to close #224. In addition, this eliminates redundant line detection logic, so #225 is also closed by this as well.
This logic seems inverted, no? If end of selection is at last step of the paragraph, there should be no further step possible, so `stepIterator.nextStep()` return `false`.
Possibly worth a unit test :smile_cat: