gh-106320: Remove private _PyUnicode_AsString() alias (#107021)

Remove private _PyUnicode_AsString() alias to PyUnicode_AsUTF8(). It
was kept for backward compatibility with Python 3.0 - 3.2.

The PyUnicode_AsUTF8() is available since Python
3.3. The PyUnicode_AsUTF8String() function can be used to keep
compatibility with Python 3.2 and older.