Apply RI trigger skipping tests also for DELETE
The tests added in
cfa0f4255bb0f5550d37a01c4d8fe2966d20040c to skip
firing an RI trigger if any old key value is NULL can also be applied
for DELETE. This should give a performance gain in those cases, and it
also saves a lot of duplicate code in the actual RI triggers. (That
code was already dead code for the UPDATE cases.)
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>