remove recipe for obsolete object-registry

The author of `object-registry' (me) has deprecated [1] it half a
year ago.  It likely is broken on Emacs 24.4, due to incompatible
changes to `eieio'.  It was only downloaded 58 times and no other
package depends on it [2].

[1] https://github.com/emacsattic/object-registry/commit/1f2185f5e6b629f59542f2e093eaf97edde8a1e3
[2] http://melpa.org/#/object-registry
Merge pull request #2164 from tarsius/remove_object-registry

remove recipe for obsolete object-registry