DB: move index from schema.sql to constraints.sql

As noted in the comment this is how it should be.

Signed-off-by: Shawn Kwang <kwangs@uwm.edu>

db/constraints.sql

@@ -179,6 +179,9 @@ alter table sent_email
     add index sent_email_userid(userid);
         -- for delete account
 
+alter table private_messages
+    add index userid(userid);
+
 alter table consent
     add index userid_ctid(userid, consent_type_id);
     add index consent_timestamp(consent_time);

db/schema.sql

@@ -627,8 +627,7 @@ create table private_messages (
     opened                  tinyint         not null default 0,
     subject                 varchar(255)    not null,
     content                 text            not null,
-    primary key(id),
-    key userid (userid)
+    primary key(id)
 ) engine=MyISAM;
 
 create table credited_job (