SPDX: Not BSD-4-Clause

This is not BSD-4-Clause. It's closer to a modified BSD-2-Clause with 2
added clauses (and the first one has added clauses). Remove
SPDX-License-Idnetifier since this license doesn't match anything in
SPDX.

sys/kern/kern_physio.c

@@ -1,6 +1,4 @@
 /*-
- * SPDX-License-Identifier: BSD-4-Clause
- *
  * Copyright (c) 1994 John S. Dyson
  * All rights reserved.
  *