Remove some more dead-code from old split impl

Removed moveTasksToFullscreenStack (only a small piece of it
was used by pip, so just localized it).

Remove stack creation/reparenting from resizeTask since modes
don't work like that anymore.

Removed some always-false parameters in AS.setWindowingMode

Bug: 133381284
Test: tests pass
Change-Id: I22e846250196eb93b480a05bed1f66d34b51a092