pgsql: Temporarily revert test case from a2b70c89ca1a5fcf6181d3c777d82e

Temporarily revert test case from a2b70c89ca1a5fcf6181d3c777d82e7b83d2de1b.

That code patch was good as far as it went, but the associated test case
has exposed fundamental brain damage in the parallel scan mechanism,
which is going to take nontrivial work to correct. In the interests of
getting the buildfarm back to green so that unrelated work can proceed,
let's temporarily remove the test case.

Branch
------
REL_10_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/9b644745c94944de8f23449524671601d8863830

Modified Files
--------------
src/test/regress/expected/select_parallel.out | 43 ---------------------------
src/test/regress/sql/select_parallel.sql | 16 ----------
2 files changed, 59 deletions(-)