z-axis: misc changes

z-axis: misc changes

* Clear hole for rotor in motor holder

* Motor holder now uses four instead of three screws, seems to make z-axis
  slightly more stable

* Removed material from back of extrusion for top rod holder, saves
  material if using corner brackets to hold the frame
  together (recommended, frame rigidity is improved enough to suffer
  another few vitamins, and they are inexpensive).

* Attempt at easier to remove support for leadscrew hole. Likely
  better solution would be to just use slicer support (build plate
  only).