Commit a08064492 ("call-list: port mm_call_list_delete_call to use GTask")
ported most of the mm_call_list_delete_call code to use GTask, but missed a
g_simple_async_report_error_in_idle calls, which can potentially lead to
an incorrect G_TASK cast in mm_call_list_delete_call_finish.
When calling g_list_free_full() to free a GList in dispose(), it is
necessary to reset the GList pointer to NULL as dispose() may be called
more than once.
g_free and g_object_unref are in form of `void (*)(gpointer)`, which
matches the GDestroyNotify signature. An explicit GDestroyNotify cast on
g_free and g_object_unref is thus not needed.
