[notebook] Doesn't use plugin as source of templates

[mirekdlugosz]

Notebook tag contains couple of templates for various IPython versions; however, new-ish IPython seems to resolve template names relative to one of many pre-specified directories, and liquid_tags plugin directory is not on that list.

Relevant code has been commented out (see pelican/plugins/liquid_tags/notebook.py line 330) to allow tag to work at all, but support for templates should be brought back.