Foreign Key written as a trigger

I have a database that has been upgrade numerous times (Started about
7.1 and is not 7.4.8). The table has 2 foreign keys on it, but when I
look at the table I see that the FK applied before 7.4.X is written as a
trigger. Everything seems to work, I was just wondering if there are
any performance reasons to change the trigger to a FK.

issp=> \d notpoll
Table "public.notpoll"
Column | Type | Modifiers
-------------+---------+-----------
notpollid | integer | not null
pollgrpid | integer |
notgroupsid | integer |
Indexes:
"notpoll_pkey" primary key, btree (notpollid)
"notpoll_pollgrpid_key" unique, btree (pollgrpid)
Foreign-key constraints:
"pollgrp_exists" FOREIGN KEY (pollgrpid) REFERENCES
pollgrpinfo(pollgrpid) ON DELETE CASCADE
Triggers:
"RI_ConstraintTrigger_2137396" AFTER INSERT OR UPDATE ON notpoll
FROM notgroups NOT DEFERRABLE INITIALLY IMMEDIATE FOR EACH ROW EXECUTE
PROCEDURE "RI_FKey_check_ins"('notgroups_exists', 'notpoll',
'notgroups', 'UNSPECIFIED', 'notgroupsid', 'notgroupsid')

Thanks,
Woody

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