gh-105156: Cleanup usage of old Py_UNICODE type (#105158)

* refcounts.dat:

  * Remove Py_UNICODE functions.
  * Replace Py_UNICODE argument type with wchar_t.

* _PyUnicode_ToLowercase(), _PyUnicode_ToUppercase(),
  _PyUnicode_ToTitlecase() are no longer deprecated in comments.
  It's no longer needed since they now use Py_UCS4 type, rather than
  the deprecated Py_UNICODE type.
* gdb: Remove unused char_width() method.