Move the system catalog index declarations from catalog/indexing.h to
the respective parent tables' catalog/pg_*.h files.  The original
reason for having it split was that the old genbki system produced the
output in the order of the catalog files it read, so all the indexing
stuff needed to come separately.  But this is no longer the case, and
keeping it together makes more sense.
Reviewed-by: John Naylor <john.naylor@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/flat/c7cc82d6-f976-75d6-2e3e-b03d2cab26bb@2ndquadrant.com