Use locale encoding if Py_FileSystemDefaultEncoding is not set

* PyUnicode_EncodeFSDefault(), PyUnicode_DecodeFSDefaultAndSize() and
   PyUnicode_DecodeFSDefault() use the locale encoding instead of UTF-8 if
   Py_FileSystemDefaultEncoding is NULL
 * redecode_filenames() functions and _Py_code_object_list (issue #9630)
   are no more needed: remove them