Remove one memoize test case added by commit
069d0ff02.
This test case turns out to depend on the assumption that a non-Var
subquery output that's underneath an outer join will always get
wrapped in a PlaceHolderVar. But that behavior causes performance
regressions in some cases compared to what happened before v16.
The next commit will avoid inserting a PHV in the same cases where
pre-v16 did, and that causes get_memoized_path to not detect that
a memoize plan could be used.
Commit this separately, in hopes that we can restore the test after
making get_memoized_path smarter. (It's failing to find memoize
plans in adjacent cases where no PHV was ever inserted, so there
is definitely room for improvement there.)
Discussion: https://postgr.es/m/CAG1ps1xvnTZceKK24OUfMKLPvDP2vjT-d+F2AOCWbw_v3KeEgg@mail.gmail.com