SPDX: Not BSD-4-Clause

This license has 4 clauses, and shares some text with the BSD-4-Clause
license. However, it omits the standard disclaimer and has 2 clauses all
its own. Remove this tag, since it was made in error and this doesn't
match the SPDX copy of the BSD-4-Clause license.

Sponsored by:		Netflix

sys/kern/sys_pipe.c

@@ -1,6 +1,4 @@
 /*-
- * SPDX-License-Identifier: BSD-4-Clause
- *
  * Copyright (c) 1996 John S. Dyson
  * Copyright (c) 2012 Giovanni Trematerra
  * All rights reserved.