Retire valuespace and jinjarender plugins

[edreamleo]

Remove leo/plugins/valuespace.py

This plugin:

• Is security hazard,
• Is overly complex and almost useless.
• Is a product of that "IPython madness" of 2008.

Remove leo/plugins/jinjarender.py

This plugin:

• Depends on the valuespace plugin.
• Is overly complex and almost useless.

Remove leo/external/stringlist.py

• Only the valuespace plugin uses stringlist.py