This is a replacement for 4cad2534. Instead of projecting all tuples
going into a HashAgg, only remove unnecessary attributes when actually
spilling. This avoids the regression for the in-memory case.

Discussion: https://postgr.es/m/a2fb7dfeb4f50aa0a123e42151ee3013933cb802.camel%40j-davis.com
Backpatch-through: 13