tests/qtest: Fix build error

Newer gcc compiler (version 16.0.0 20260103 (Red Hat 16.0.0-0) (GCC))
detects an unused variable error:

  ../tests/qtest/libqtest.c: In function ‘qtest_qom_has_concrete_type’:
  ../tests/qtest/libqtest.c:1044:9: error: variable ‘idx’ set but not used [-Werror=unused-but-set-variable=]

Remove idx.

Cc: Fabiano Rosas <farosas@suse.de>
Cc: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Link: https://lore.kernel.org/qemu-devel/20260112123146.1010621-1-clg@redhat.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>

tests/qtest/libqtest.c

@@ -1042,7 +1042,6 @@ static bool qtest_qom_has_concrete_type(const char *parent_typename,
     QObject *qobj;
     QString *qstr;
     QDict *devinfo;
-    int idx;
 
     if (!list) {
         QDict *resp;
@@ -1067,7 +1066,7 @@ static bool qtest_qom_has_concrete_type(const char *parent_typename,
         }
     }
 
-    for (p = qlist_first(list), idx = 0; p; p = qlist_next(p), idx++) {
+    for (p = qlist_first(list); p; p = qlist_next(p)) {
         devinfo = qobject_to(QDict, qlist_entry_obj(p));
         g_assert(devinfo);