nautilus-canvas-container: Remove the "include visible area" logic.
canvas_set_scroll_region_include_visible_area() was added in ec054c80 to fix https://bugzilla.gnome.org/show_bug.cgi?id=42068 ("Dragging icons adjusts scroll area in a way that causes immediate scrolling"). This is no longer the case, because now icons remain in place when being dragged and are not allowed to be rearranged.
ec054c80 causes issues relating to extra scrolling space (#340 (closed)), hence the removal.
Fixes #340 (closed).