Fix issue ASKBOT#845

* ASKBOT#845
* setup_aggregates() used to conditionally
        DROP AGGREGATE concat_tsvectors(tsvector);
  - the condition SELECTs a field from an internal table that
    was removed in Postgresql 11, consequently the function
    definition, therefore the script, therefore the django
    migration fails
  - the problematic code section is a custom implementation of
    DROP AGGREGATE IF EXISTS
  - DROP AGGREGATE IF EXSITS has been supported (at least) since
    Postgres 8.2.23, released 2006
  => replaced custom implementation with DROP AGGREGATE IF EXISTS
* this patch removes the reference to the changed internal Postgresql
  table and therefore the issue which makes the script fail using
  Postgresql 11

askbot/search/postgresql/thread_and_post_models_03012016.plsql

@@ -48,12 +48,8 @@ $$ LANGUAGE plpgsql;
 
 CREATE OR REPLACE FUNCTION setup_aggregates() RETURNS boolean AS
 $$
-DECLARE
-    onerow record;
 BEGIN
-    FOR onerow IN SELECT * FROM pg_proc WHERE proname = 'concat_tsvectors' AND proisagg LOOP
-        DROP AGGREGATE concat_tsvectors(tsvector);
-    END LOOP;
+    DROP AGGREGATE IF EXISTS concat_tsvectors(tsvector);
     CREATE AGGREGATE concat_tsvectors (
         BASETYPE = tsvector,
         SFUNC = tsv_add,