Disable GROUP BY Pushdown by default

The mentioned GROUP BY pushdown proved to be error-prone, e.g. its execution starts after optimize phase so MDB optimizer might has rewritten JOIN conditions dropping some of them. The optimized conditions forces the query to error-out. Moreover this pushdown method are rarely called b/c it is the last in the precedence of pushdown methods available:

• select handler
• derived handler
• group by handler
• table scan with conditions
  Deactivation of GBH might implicitly affects the variety of queries that MCS can process so at least full test001 should be run with the patch.