optimzed SQL query in hotpot_get_participants to return only user ids…

… (not whole user record)

mod/hotpot/lib.php

@@ -1123,15 +1123,15 @@ function hotpot_get_precision(&$hotpot) {
 }
 
 function hotpot_get_participants($hotpotid) {
-//Must return an array of user records (all data) who are participants
+//Must return an array of user ids who are participants
 //for a given instance of hotpot. Must include every user involved
 //in the instance, independient of his role (student, teacher, admin...)
 //See other modules as example.
 	global $CFG;
 
 	return get_records_sql("
 		SELECT DISTINCT 
-			u.*
+			u.id, u.id
 		FROM 
 			{$CFG->prefix}user u,
 			{$CFG->prefix}hotpot_attempts a