This reverts commit 9e108984, after finding out that buildfarm members
running SLES 15 on z390 complain on the compression and decompression
logic of the new test: pipistrelles, barbthroat and steamerduck.

Those hosts are visibly using hardware-specific changes to improve zlib
performance, requiring more investigation.

Thanks to Tom Lane for the discussion.

Discussion: https://postgr.es/m/20200722093749.GA2564@paquier.xyz
Backpatch-through: 9.5