Remove user_id and remote_user_id constraints in v4&v5 migrations

It's not straightforward to remove these constraints in SQLite, so this
just skips it entirely. Since both of these migrations are part of the
same WF release, this should have minimal impact on admins.

migrations/v4.go

@@ -18,8 +18,6 @@ func oauth(db *datastore) error {
 			SetIfNotExists(true).
 			Column(dialect.Column("user_id", wf_db.ColumnTypeInteger, wf_db.UnsetSize)).
 			Column(dialect.Column("remote_user_id", wf_db.ColumnTypeInteger, wf_db.UnsetSize)).
-			UniqueConstraint("user_id").
-			UniqueConstraint("remote_user_id").
 			ToSQL()
 		if err != nil {
 			return err

migrations/v5.go

@@ -49,8 +49,6 @@ func oauthSlack(db *datastore) error {
 						"access_token",
 						wf_db.ColumnTypeVarChar,
 						wf_db.OptionalInt{Set: true, Value: 512,})),
-			dialect.DropIndex("remote_user_id", "oauth_users"),
-			dialect.DropIndex("user_id", "oauth_users"),
 			dialect.CreateUniqueIndex("oauth_users", "oauth_users", "user_id", "provider", "client_id"),
 		}
 		for _, builder := range builders {