Remove WITHOUT_GNU and WITHOUT_GNU_SUPPORT src.conf.knobs

Remove WITHOUT_GNU and WITHOUT_GNU_SUPPORT src.conf.knobs

These have no effect (and WITHOUT_GNU is documented as having no
effect). I intend to later introduce a WITHOUT_GPL knob to serve a
similar purpose as WITHOUT_GNU's previously documented intent, but with
a more accurate name. To avoid confusion over the transition though just
remove the existing, nonfunctional ones.