We have a perfectly good convention for OID macros for built-in functions
already, so making custom symbols is just introducing unnecessary
deviation from the convention.  Remove the one case that had snuck in,
and add an error check in genbki.pl to discourage future instances.

Although this touches pg_proc.dat, there's no need for a catversion
bump since the actual catalog data isn't changed.

John Naylor

Discussion: https://postgr.es/m/CAFBsxsHpCbjfoddNGpnnnY5pHwckWfiYkMYSF74PmP1su0+ZOw@mail.gmail.com