The "emulation" I wrote for PQsendQuery in pipeline mode to use extended
query protocol, in commit acb7e4eb, 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