1db689715 added debugging output to the partition_prune regression test
to help us figure out why buildfarm member Parula was occasionally
failing.
We've not seen any failures in around 4 weeks and the best guess as to
what the problem was is a compiler bug. Since there are no recent
failures, there's now no need to keep this debugging code, so revert it.
Discussion: https://postgr.es/m/CAApHDvqyLF881EvDtXT=ossa0i4ioJBtW2c0Wbouzt5d3HDb5Q@mail.gmail.com
ec1a405adc