refactor: remove redundant buf_owner from _get_buffer_or_str

Since str auto-encoding was removed, *owner is always NULL. Removed
the owner parameter from _get_buffer_or_str and eliminated all
associated buf_owner declarations, Py_XDECREF calls, and cleanup
logic across _parse_fastcall_args, module functions, vectorcall,
tp_init, and update().

Author: ifduyue