Remove an unrelevant assert.

sql/sql_select.cc:
  This assert is not relevant because:
  - the correct assert is DBUG_ASSERT(! (full && sl->uncacheable)) 
    (prevents freeing of uncacheable JOINs), it breaks view.test
  - it seems we can free internal JOINs, even if they are uncacheable:
    if the top level join is evaluated, we're not going to need  the
    internal joins any more

sql/sql_select.cc

@@ -5912,13 +5912,7 @@ void JOIN::join_free(bool full)
     {
       JOIN *join= sl->join;
       if (join)
-      {
-        /* Check that we don't occasionally clean up an uncacheable JOIN */
-#if 0
-        DBUG_ASSERT(! (!select_lex->uncacheable && sl->uncacheable));
-#endif
         join->join_free(full);
-      }
     }
 
   /*