Remove some unnecessary fields from executor nodes.

Remove some unnecessary fields from executor nodes.

JsonExprState.input_finfo is only assigned to, never read, and
it's really fairly useless since the value can be gotten out of
the adjacent input_fcinfo field.  Let's remove it before someone
starts to depend on it.

While here, also remove TidScanState.tss_htup and AggState.combinedproj,
which are referenced nowhere.  Those should have been removed by the
commits that caused them to become disused, but were not.

I don't think a catversion bump is necessary here, since plan trees
are never stored on disk.

Matthias van de Meent

Discussion: https://postgr.es/m/CAEze2WjsY4d0TBymLNGK4zpttUcg_YZaTjyWz2VfDUV6YH8wXQ@mail.gmail.com