Remove add-ons mechanism

[Joseph Myers]

On Thu, 5 Oct 2017, Florian Weimer wrote:

> Their packaging is completely different from the libidn upstream releases.

And updating from the libidn upstream releases would be problematic given 
that it's a non-FSF-assigned project that has changed license.  (See 
however the idea of enabling glibc to dlopen unmodified upstream libidn to 
implement the functionality that uses glibc's copy of libidn, as a 
possible replacement of that copy.)

-- 
Joseph S. Myers
joseph@codesourcery.com