Remove ExternalUser authentication code

This was an experimental authentication system intoduced a couple
of years ago with a pretty narrow use-case. It's been pretty much
ignored since introduction, and makes login more complicated than
it needs to be.

I didn't drop the external_user table on the off-chance someone
out there actually has data in it, but they should use AuthPlugin
for their external authentication needs.

Change-Id: I794338dbb75961ee033d41fa44bb7aa22e54f447