BUG#884631: Table elimination works 5.3 release builds even if turned off

- Make table elimination to actually switch itself on/off in release builds.

mysql-test/t/table_elim_debug.test

 #
 # Table elimination (MWL#17) tests that need debug build
 #
---source include/have_debug.inc
+# In MariaDB 5.3, one can switch table_elimination on/off without debug, too: 
+## --source include/have_debug.inc
 
 --disable_warnings
 drop table if exists t1, t2;

sql/opt_table_elimination.cc

@@ -588,10 +588,8 @@ void eliminate_tables(JOIN *join)
   if (!join->outer_join)
     DBUG_VOID_RETURN;
 
-#ifndef DBUG_OFF
   if (!optimizer_flag(thd, OPTIMIZER_SWITCH_TABLE_ELIMINATION))
     DBUG_VOID_RETURN; /* purecov: inspected */
-#endif
 
   /* Find the tables that are referred to from WHERE/HAVING */
   used_tables= (join->conds?  join->conds->used_tables() : 0) |