pgsql: Revert "Fix accumulation of parallel worker instrumentation."

Revert "Fix accumulation of parallel worker instrumentation."

This reverts commit 2c09a5c12a66087218c7f8cba269cd3de51b9b82. Per
further discussion, that doesn't seem to be the best possible fix.

Discussion: http://postgr.es/m/CAA4eK1LW2aFKzY3=vwvc=t-juzPPVWP2uT1bpx_MeyEqnM+p8g@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1d6fb35ad62968c8678d0321887e2b9ca8fe1a84

Modified Files
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src/backend/executor/execParallel.c | 51 +++++++--------------------
src/test/regress/expected/select_parallel.out | 21 -----------
src/test/regress/sql/select_parallel.sql | 7 ----
3 files changed, 13 insertions(+), 66 deletions(-)