Twenty years ago, we removed certain operator classes in favor of
letting indexes over their data types be built with some other
binary-compatible, more standard opclass.  As a hack to allow existing
index definitions to be dumped and reloaded, we made CREATE INDEX ignore
the removed opclass names, so that such indexes would fall back to the
new default opclass for their data types.  This was never intended to
be a long-lived thing; it carries the obvious risk of breaking some
future developer's attempt to re-use those old opclass names.  Since
all of the cases in question are for opclasses that were removed
before PG 8.0, it seems okay to get rid of these hacks now.

This is part of a group of patches removing various server-side kluges
for transparently upgrading pre-8.0 dump files.  Since we've had few
complaints about dropping pg_dump's support for dumping from pre-8.0
servers (commit 64f3524e), it seems okay to now remove these kluges.

Discussion: https://postgr.es/m/3685.1583422389@sss.pgh.pa.us