MNT: do not refer to the optional data packages

The rationale is that script `doc/source/devel/register_me.py`
is a Python 2 script that is not compatible with Python 3.
Looks like the whole machinery has not been used for ages.

Author: DimitriPapadopoulos

doc/source/devel/data_pkg_design.rst

@@ -21,7 +21,6 @@ progress.
 
     spm_use
     modified_images
-    data_pkg_design
     data_pkg_discuss
     data_pkg_uses
     scaling