Avoid unnecessary overhead during connection reset (#648)

UNLISTEN is now available in Hot Standby mode in all supported
PostgreSQL versions, therefore there's no reason anymore to wrap
it in DO block. This should significantly speed up connection reset.

asyncpg/connection.py

@@ -1501,16 +1501,7 @@ def _get_reset_query(self):
         if caps.sql_close_all:
             _reset_query.append('CLOSE ALL;')
         if caps.notifications and caps.plpgsql:
-            _reset_query.append('''
-                DO $$
-                BEGIN
-                    PERFORM * FROM pg_listening_channels() LIMIT 1;
-                    IF FOUND THEN
-                        UNLISTEN *;
-                    END IF;
-                END;
-                $$;
-            ''')
+            _reset_query.append('UNLISTEN *;')
         if caps.sql_reset:
             _reset_query.append('RESET ALL;')