MDEV-8816 Equal field propagation is not applied for WHERE varbinary_column>=_utf8'a' COLLATE utf8_general_ci AND varbinary_column='A';
1. Removing the legacy code that disabled equal field propagation in cases
   when comparison is done as VARBINARY. This is now correctly handled by
   the new propagation code in Item_xxx::propagate_equal_fields() and
   Field_str::can_be_substituted_to_equal_item (the bug fix).
2. Also, removing legacy (pre-MySQL-4.1) Arg_comparator methods
   compare_binary_string() and compare_e_binary_string(), as VARBINARY
   comparison is correcty handled in compare_string() and compare_e_string() by
   the corresponding VARBINARY collation handler implemented in my_charset_bin.
   (not really a part of the bug fix)