usr.bin/ar: remove incorrect SPDX tags

Three ar files have a non-standard variation of the BSD license, so
remove their SPDX tags.

Sponsored by:	The FreeBSD Foundation

usr.bin/ar/ar.c

@@ -1,6 +1,4 @@
 /*-
- * SPDX-License-Identifier: BSD-3-Clause
- *
  * Copyright (c) 2007 Kai Wang
  * Copyright (c) 2007 Tim Kientzle
  * Copyright (c) 2007 Joseph Koshy

usr.bin/ar/read.c

@@ -1,6 +1,4 @@
 /*-
- * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
- *
  * Copyright (c) 2007 Kai Wang
  * Copyright (c) 2007 Tim Kientzle
  * All rights reserved.

usr.bin/ar/util.c

@@ -1,6 +1,4 @@
 /*-
- * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
- *
  * Copyright (c) 2003-2007 Tim Kientzle
  * All rights reserved.
  *