editors/emacs-devel: Remove JSON option

Upstream introduced a new, faster JSON encoder that does not depend on
any external library and is always on.

https://git.savannah.gnu.org/cgit/emacs.git/commit/?id=890edfd2bb8fd79730919972cc82811b73c7f572

Reported by: Pedro Andres Aranda Gutierrez <paaguti@gmail.com>