Fix oversight in outer join removal.

Fix oversight in outer join removal.

A placeholder that references the outer join's relid in ph_eval_at
is logically "above" the join, and therefore we can't remove its
PlaceHolderInfo: it might still be used somewhere in the query.

This was not an issue pre-v16 because we failed to remove the join
at all in such cases.  The new outer-join-aware-Var infrastructure
permits deducing that it's okay to remove the join, but then we
have to clean up correctly afterwards.

Report and fix by Richard Guo

Discussion: https://postgr.es/m/CAMbWs4_tuVn9EwwMcggGiZJWWstdXX_ci8FeEU17vs+4nLgw3w@mail.gmail.com