Get rid of Item_func::select_optimize() and Item_func::optimize_type

After a set of patches made under terms MDEV-7950, Item_func::select_optimize() and Item_func::optimize_type have now a very little sense.

select_optimize() is now used only in Item_func::get_mm_tree().

We just need to split Item_func::get_mm_tree() into:

• a non-optimized default implementation for Item_func::get_mm_tree(),
• an optimized implemetation for Item_bool_rowready_func2::get_mm_tree()
• an optionally optimized implementation in Item_func_like::get_mm_tree().