Remove ancient support for upgrading pre-7.3 foreign key constraints.

Before 7.3, foreign key constraints had no explicit catalog
representation, so that what pg_dump produced for them was (usually)
a set of three CREATE CONSTRAINT TRIGGER commands.  Commit a2899ebd
and some follow-on fixes added an ugly hack in CreateTrigger() to
recognize that pattern and reconstruct the foreign key definition.
However, we've never had any test coverage for that code, so that it's
legitimate to wonder if it still works; and having to maintain it in
the face of upcoming trigger-related patches seems rather pointless.
Let's decree that its time has passed, and drop it.

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.

Daniel Gustafsson

Discussion: https://postgr.es/m/805874E2-999C-4CDA-856F-1AFBD9DFE933@yesql.se