MDEV-8360 Clean-up CHARSET_INFO: strnncollsp: diff_if_only_endspace_difference

- Removing the "diff_if_only_endspace_difference" argument from
  MY_COLLATION_HANDLER::strnncollsp(), my_strnncollsp_simple(),
  as well as in the function template MY_FUNCTION_NAME(strnncollsp)
  in strcoll.ic

- Removing the "diff_if_only_space_different" from ha_compare_text(),
  hp_rec_key_cmp().

- Adding a new function my_strnncollsp_padspace_bin() and reusing
  it instead of duplicate code pieces in my_strnncollsp_8bit_bin(),
  my_strnncollsp_latin1_de(), my_strnncollsp_tis620(),
  my_strnncollsp_utf8_cs().

- Adding more tests for better coverage of the trailing space handling.

- Removing the unused definition of HA_END_SPACE_ARE_EQUAL

sql/sql_select.cc

@@ -16725,8 +16725,6 @@ create_tmp_table(THD *thd, TMP_TABLE_PARAM *param, List<Item> &fields,
           cur_group->buff++;                        // Pointer to field data
 	  group_buff++;                         // Skipp null flag
 	}
-        /* In GROUP BY 'a' and 'a ' are equal for VARCHAR fields */
-        key_part_info->key_part_flag|= HA_END_SPACE_ARE_EQUAL;
 	group_buff+= cur_group->field->pack_length();
       }
       keyinfo->key_length+=  key_part_info->length;