pgsql: Drop SERIALIZABLE workaround from parallel query tests.

Drop SERIALIZABLE workaround from parallel query tests.

SERIALIZABLE no longer inhibits parallelism, so we can drop some
outdated workarounds and comments from regression tests. The change
came in release 12, commit bb16aba5, but it's not really worth
back-patching.

Also fix a typo.

Reviewed-by: Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com>
Discussion: https://postgr.es/m/CA%2BhUKGJUaHeK%3DHLATxF1JOKDjKJVrBKA-zmbPAebOM0Se2FQRg%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/eeb60e45d82d5840b713a8741ae552238d57e8b9

Modified Files
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src/test/regress/expected/aggregates.out | 4 ++--
src/test/regress/expected/explain.out | 4 +---
src/test/regress/expected/insert_parallel.out | 4 +---
src/test/regress/expected/select_parallel.out | 4 +---
src/test/regress/expected/write_parallel.out | 6 ++----
src/test/regress/sql/aggregates.sql | 4 ++--
src/test/regress/sql/explain.sql | 4 +---
src/test/regress/sql/insert_parallel.sql | 4 +---
src/test/regress/sql/select_parallel.sql | 4 +---
src/test/regress/sql/write_parallel.sql | 6 ++----
10 files changed, 14 insertions(+), 30 deletions(-)