Remove negative NULL-check warning

According to the docs in the GSourceFuncs, the timeout_ parameter can be
NULL. But when it's NULL, it would be equivalent to an invocation to
return -1 as timeout_ and FALSE as return value. This is exactly the
same, what we're doing if timeout_ is not NULL.

src/x11/x.c

@@ -831,8 +831,6 @@ gboolean x_mainloop_fd_prepare(GSource *source, gint *timeout)
 {
         if (timeout)
                 *timeout = -1;
-        else
-                LOG_E("x_mainloop_fd_prepare: timeout == NULL");
         return false;
 }