Revert "Rewrite some RI code to avoid using SPI"
This reverts commit
99392cdd78b788295e52b9f4942fa11992fd5ba9.
We'd rather rewrite ri_triggers.c as a whole rather than piecemeal.
Discussion: https://postgr.es/m/E1ncXX2-000mFt-Pe@gemulon.postgresql.org