Revert "Replace most GObject warnings with criticals"

This reverts commit 0ffe86a1f7.

This was intended to land for the 2.75.x unstable series, and not in the
2.74.x stable series.

Fixes: #2788

gio/tests/glistmodel.c

@@ -73,7 +73,7 @@ test_store_non_gobjects (void)
 
   g_test_trap_subprocess (NULL, 0, G_TEST_SUBPROCESS_DEFAULT);
   g_test_trap_assert_failed ();
-  g_test_trap_assert_stderr ("*CRITICAL*value * of type 'GType' is invalid or "
+  g_test_trap_assert_stderr ("*WARNING*value * of type 'GType' is invalid or "
                              "out of range for property 'item-type'*");
 }