pgsql: Revert "Allow parallel workers to cope with a newly-created sess

Revert "Allow parallel workers to cope with a newly-created session user ID."

This reverts commit 48536305370acd75c6264fcefec7fae7af8c5440.

Some buildfarm animals are failing with "cannot change
"client_encoding" during a parallel operation". It looks like
assign_client_encoding is unhappy at being asked to roll back a
client_encoding setting after a parallel worker encounters a
failure. There must be more to it though: why didn't I see this
during local testing? In any case, it's clear that moving the
RestoreGUCState() call is not as side-effect-free as I thought.
Given that the bug f5f30c22e intended to fix has gone unreported
for years, it's not something that's urgent to fix; I'm not
willing to risk messing with it further with only days to our
next release wrap.

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/afbf32fa1aa87f710b623cbdf746c55b4799574e

Modified Files
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src/backend/access/transam/parallel.c | 12 ++++--------
src/backend/commands/variable.c | 10 ++--------
src/test/regress/expected/select_parallel.out | 18 ------------------
src/test/regress/sql/select_parallel.sql | 9 ---------
4 files changed, 6 insertions(+), 43 deletions(-)