It wasn't very useful. ORCA and Postgres both just stack WindowAgg nodes
on top of each other, and no-one's been unhappy about that, so we might as
well do that, too. This reduces the difference between GPDB and the upstream
implementation, and will hopefully make it smoother to switch.

Rename the Window Plan node type to WindowAgg, to match upstream, now
that it is fairly close to the upstream version.