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

strings/strcoll.ic

@@ -210,17 +210,13 @@ MY_FUNCTION_NAME(strnncoll)(CHARSET_INFO *cs __attribute__((unused)),
   @param a_length    - the length of the left string
   @param b           - the right string
   @param b_length    - the length of the right string
-  @param diff_if_only_endspace_difference - not used in the code.
-                       TODO: this should be eventually removed (in 10.2?)
   @return            - the comparison result
 */
 
 static int
 MY_FUNCTION_NAME(strnncollsp)(CHARSET_INFO *cs __attribute__((unused)),
                               const uchar *a, size_t a_length, 
-                              const uchar *b, size_t b_length,
-                              my_bool diff_if_only_endspace_difference
-                              __attribute__((unused)))
+                              const uchar *b, size_t b_length)
 {
   const uchar *a_end= a + a_length;
   const uchar *b_end= b + b_length;