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outdated workarounds and comments from regression tests.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.rupireddyforpostgres@gmail.com>
Discussion: https://postgr.es/m/CA%2BhUKGJUaHeK%3DHLATxF1JOKDjKJVrBKA-zmbPAebOM0Se2FQRg%40mail.gmail.com