Remove the flaky check in event_trigger_login regression test

Remove the flaky check in event_trigger_login regression test

The query checks that pg_database.dathasloginevt is unset on connect when
there are no event triggers.  However, unsetting this flag is implemented in
a non-blocking way, so a concurrent autovacuum connection breaks this check.
It doesn't seem we can do much with this, at least within a regression test.
So, remove it.

Reported-by: Alexander Lakhin
Discussion: https://postgr.es/m/44807d19-81a6-3884-3e0f-22dd99aac3ed%40gmail.com