Revert "gio/tests/cancellable: Explain failure on GCancellableSource tests on valgrind"

This reverts commit 365411ea32.

Since commit 3a07b2abd4, issue 2309 has
been fixed, so we should no longer have failures from valgrind here.

gio/tests/cancellable.c

@@ -25,7 +25,6 @@
 #include <gio/gio.h>
 
 #include "glib/glib-private.h"
-#include "glib/gvalgrind.h"
 
 /* How long to wait in ms for each iteration */
 #define WAIT_ITERATION (10)
@@ -276,11 +275,6 @@ test_cancellable_source_threaded_dispose (void)
   g_test_bug ("https://gitlab.gnome.org/GNOME/glib/issues/2309");
 #endif
 
-#ifdef ENABLE_VALGRIND
-  if (RUNNING_ON_VALGRIND)
-    g_test_incomplete ("FIXME: Leaks lots of GCancellableSource objects, see glib#2309");
-#endif
-
   /* Create a new thread and wait until it’s ready to execute. Each iteration of
    * the test will pass it a new #GCancellableSource. */
   g_cond_init (&data.cond);