Removing similar functions from ctype-utf8.c and ctype-ucs2.c

- my_tosort_utf16()
- my_tosort_utf32()
- my_tosort_ucs2()
- my_tosort_unicode()

Adding new shared functions into ctype-unidata.h:

- my_tosort_unicode_bmp()  - reused for utf8mb3, ucs2
- my_tosort_unicode()      - reused for utf8mb4, utf16, utf32

For simplicity, the new version of my_tosort_unicode*()
does not include the code handling the MY_CS_LOWER_SORT flag because:
- it affects performance negatively
- we don't have any collations with this flag yet anyway
(This code was most likely earlier erroneously merged from
MySQL's utf8_tolower_ci at some point.)