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.

doc/reference/clutter/Makefile.am

@@ -71,7 +71,6 @@ IGNORE_HFILES=\
 	clutter-deprecated.h 		\
 	clutter-enum-types.h 		\
 	clutter-id-pool.h 		\
-	clutter-json.h 			\
 	clutter-keysyms.h 		\
 	clutter-keysyms-compat.h	\
 	clutter-keysyms-table.h 	\
@@ -88,7 +87,6 @@ IGNORE_HFILES=\
 	egl 				\
 	fruity				\
 	glx 				\
-	json 				\
 	osx 				\
 	x11 				\
 	win32