Remove cache of last stacking order in stack.c

stack.c kept it's own record of the last stacking it requested, so that
restacking could be done with minimal moves, but we already have a better
view of the stacking order with the stack tracker, so use that instead.

This allows eliminating the special case for the first restack.

https://bugzilla.gnome.org/show_bug.cgi?id=736559