Remove the internal copy of JSON-GLib

The internal copy of JSON-GLib was meant to go away right after the 1.0
release, given that JSON-GLib was still young and relatively unknown.

Nowadays, many projects started depending on this little library, and
distributions ship it and keep it up to date.

Keeping a copy of JSON-GLib means keeping it up to date; unfortunately,
this would also imply updating the code not just for the API but for the
internal implementations.

Starting with the 1.2 release, Clutter preferably dependend on the
system copy; with the 1.4 release we stopped falling back automatically.
The 1.6 cycle finally removes the internal copy and requires a copy of
JSON-GLib installed on the target system in order to compile Clutter.

clutter/clutter-script.c

@@ -210,8 +210,6 @@
 #include "clutter-private.h"
 #include "clutter-debug.h"
 
-#include "json/json-parser.h"
-
 enum
 {
   PROP_0,