(PUP-3372) Remove extlookup function

The extlookup function has been supplanted by hiera as the primary
method of looking up external data and can be replaced entirely by
hiera.
(PUP-3372) Remove extlookup from emacs keywords
Merge pull request #3205 from adrienthebo/removal/master/pup-3372-remove-extlookup

Removal/master/pup 3372 remove extlookup