Remove dead foreign key optimization code

Remove dead foreign key optimization code

The ri_KeysEqual() calls in the foreign-key trigger functions to
optimize away some updates are useless because since
adfeef55cbcc5dc72a772777f88c1be05a70dfee those triggers are not enqueued
at all.  (It's also not useful to keep these checks as some kind of
backstop, since it's also semantically correct to just run the full
check even with equal keys.)

Reviewed-by: Daniel Gustafsson <daniel@yesql.se>