Propose removing contrib/spi/preprocessor/

So while poking around contrib/spi and trying to put together an SGML
doc file for it, I realized that the preprocessor/ subdirectory seems
entirely useless. What it does is generate CREATE TRIGGER commands
for use with contrib/spi/refint.c, given SQL input that includes FOREIGN
KEY clauses. That was probably useful before we had foreign key
functionality, but I cannot imagine a use for it today --- in practice
you'd use the real FK support.

Of course, you could argue that refint.c itself has zero use anymore,
but as an example of custom trigger writing it may still have some
value, so I'm not proposing removing it. I don't, however, see the
point of keeping this preprocessor around.

regards, tom lane