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_string.cc

@@ -759,7 +759,7 @@ int sortcmp(const String *s,const String *t, CHARSET_INFO *cs)
 {
  return cs->coll->strnncollsp(cs,
                               (uchar *) s->ptr(),s->length(),
-                              (uchar *) t->ptr(),t->length(), 0);
+                              (uchar *) t->ptr(),t->length());
 }