Remove PQsendQuery support in pipeline mode

Remove PQsendQuery support in pipeline mode

The extended query protocol implementation I added in commit
acb7e4eb6b1c has bugs when used in pipeline mode.  Rather than spend
more time trying to fix it, remove that code and make the function rely
on simple query protocol only, meaning it can no longer be used in
pipeline mode.

Users can easily change their applications to use PQsendQueryParams
instead.  We leave PQsendQuery in place for Postgres 14, just in case
somebody is using it and has not hit the mentioned bugs; but we should
recommend that it not be used.

Backpatch to 15.

Per bug report from Gabriele Varrazzo.
Discussion: https://postgr.es/m/CA+mi_8ZGSQNmW6-mk_iSR4JZB_LJ4ww3suOF+1vGNs3MrLsv4g@mail.gmail.com