use normal unique (HA_NOSAME) keys for expression cache

temptables, not "uniques", that are hash-based keys.

sql/sql_expression_cache.cc:
  Don't set uniques (we don't want or need an unique
  constraint on this table)

sql/sql_expression_cache.cc

@@ -123,7 +123,6 @@ void Expression_cache_tmptable::init()
     goto error;
   }
   cache_table->s->keys= 1;
-  cache_table->s->uniques= 1;
   ref.null_rejecting= 1;
   ref.disable_cache= FALSE;
   ref.has_record= 0;