None of the other bison parsers contains this directive, and it gives
rise to some unfortunate and impenetrable messages, so just remove it.

Backpatch to release 12, where it was introduced.

Per gripe from Erik Rijkers

Discussion: https://postgr.es/m/ba069ce2-a98f-dc70-dc17-2ccf2a9bf7c7@xs4all.nl