pgsql: Stop using PQsendQuery in libpq_pipeline

Stop using PQsendQuery in libpq_pipeline

The "emulation" I wrote for PQsendQuery in pipeline mode to use extended
query protocol, in commit acb7e4eb6b1c, is problematic.  Due to numerous
bugs we'll soon remove it.  As a first step and for all branches back to
14, stop using PQsendQuery in libpq_pipeline.  Also remove a few test
lines that will no longer be relevant.

Backpatch to 14.

Discussion: https://postgr.es/m/CA+mi_8ZGSQNmW6-mk_iSR4JZB_LJ4ww3suOF+1vGNs3MrLsv4g@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d11a41a4ce79660d1a1c5f81b051061717099a9f

Modified Files
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src/test/modules/libpq_pipeline/libpq_pipeline.c   | 138 ++-------------------
.../libpq_pipeline/traces/pipeline_abort.trace     |   6 +-
.../libpq_pipeline/traces/pipeline_idle.trace      |  67 +---------
3 files changed, 13 insertions(+), 198 deletions(-)