Fix notebook rules

In #1384, I made it such that someone could run `make notebooks/py/xxxx.py` and run the notebook.  Unfortunately, that created a dependency between the rule that regenerated the .ipynb file and the notebook running rule.  This is not desirable, as regenerating the .ipynb files take forever now.

This PR breaks that dependency.  The idea is that the notebook regeneration process is so lightweight that we don't really need to examine the mtimes, and just do it blindly.

Test: make -j validate-ipynb does not run the notebooks

notebooks/subdir.mk

@@ -29,10 +29,10 @@ $(ipynb_validate_targets): validate__notebooks/%.ipynb :
 	nbencdec encode notebooks/$*.ipynb $(TEMPFILE)
 	diff -q <(cat notebooks/py/$*.py | egrep -v '^# EPY: stripped_notebook: ') <(cat $(TEMPFILE) | egrep -v '# EPY: stripped_notebook: ')
 
-$(ipynb_regenerate_targets): regenerate__notebooks/%.ipynb : notebooks/py/%.py
+$(ipynb_regenerate_targets): regenerate__notebooks/%.ipynb :
 	nbencdec decode notebooks/py/$*.py notebooks/$*.ipynb
 
-$(py_regenerate_targets): regenerate__notebooks/py/%.py : notebooks/%.ipynb
+$(py_regenerate_targets): regenerate__notebooks/py/%.py :
 	nbencdec encode notebooks/$*.ipynb notebooks/py/$*.py
 
-.PHONY : $(py_files)
+.PHONY : $(py_files) $(ipynb_regenerate_targets) $(py_regenerate_targets)