Avoid freeing objects during json aggregate finalization
Commit
f4fb45d15c tried to free memory during aggregate finalization.
This cause issues, particularly when used as a window function, so stop
doing that.
Per complaint by Jaime Casanova and diagnosis by Andres Freund
Discussion: https://postgr.es/m/YkfeMNYRCGhySKyg@ahch-to