diagnostics: remove forward decl of json::value from diagnostic.h

I believe this hasn't been necessary since r15-1413-gd3878c85f331c7.

gcc/ChangeLog:
	* diagnostic.h (json::value): Remove forward decl.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>

gcc/diagnostic.h

@@ -220,7 +220,6 @@ public:
 };
 
 class edit_context;
-namespace json { class value; }
 class diagnostic_client_data_hooks;
 class logical_location;
 class diagnostic_diagram;