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

src/test/regress/expected/event_trigger_login.out

@@ -37,9 +37,3 @@ DROP TABLE user_logins;
 DROP EVENT TRIGGER on_login_trigger;
 DROP FUNCTION on_login_proc();
 \c
-SELECT dathasloginevt FROM pg_database WHERE datname= :'DBNAME';
- dathasloginevt 
-----------------
- f
-(1 row)
-

src/test/regress/sql/event_trigger_login.sql

@@ -22,4 +22,3 @@ DROP TABLE user_logins;
 DROP EVENT TRIGGER on_login_trigger;
 DROP FUNCTION on_login_proc();
 \c
-SELECT dathasloginevt FROM pg_database WHERE datname= :'DBNAME';