JSON-GLib was added in v2.9.0rc0-201-g511c2e166a, but is no longer
necessary since we have a home-grown JSON dumper (wsutil/json_dumper.h).
Remove the remaining traces and additionally remove GObject from
FindGLIB2.cmake since it was only added for JSON-GLib.
Change-Id: If9dfd2c60cec130f98109d100bdb6618bde06ba0
Reviewed-on: https://code.wireshark.org/review/30733
Petri-Dish: Peter Wu <peter@lekensteyn.nl>
Tested-by: Petri Dish Buildbot
Reviewed-by: Anders Broman <a.broman58@gmail.com>