Cleanup dead spa_boot code

Unused code detected by coverity.

Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Neal Gompa <ngompa@datto.com>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #13868

include/Makefile.am

@@ -76,7 +76,6 @@ COMMON_H = \
 	sys/sa_impl.h \
 	sys/skein.h \
 	sys/spa.h \
-	sys/spa_boot.h \
 	sys/spa_checkpoint.h \
 	sys/spa_checksum.h \
 	sys/spa_impl.h \

include/sys/spa_boot.h

@@ -1,42 +0,0 @@
-/*
- * CDDL HEADER START
- *
- * The contents of this file are subject to the terms of the
- * Common Development and Distribution License (the "License").
- * You may not use this file except in compliance with the License.
- *
- * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
- * or https://opensource.org/licenses/CDDL-1.0.
- * See the License for the specific language governing permissions
- * and limitations under the License.
- *
- * When distributing Covered Code, include this CDDL HEADER in each
- * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
- * If applicable, add the following below this CDDL HEADER, with the
- * fields enclosed by brackets "[]" replaced with your own identifying
- * information: Portions Copyright [yyyy] [name of copyright owner]
- *
- * CDDL HEADER END
- */
-/*
- * Copyright 2009 Sun Microsystems, Inc.  All rights reserved.
- * Use is subject to license terms.
- */
-
-#ifndef _SYS_SPA_BOOT_H
-#define	_SYS_SPA_BOOT_H
-
-#include <sys/nvpair.h>
-
-#ifdef	__cplusplus
-extern "C" {
-#endif
-
-extern char *spa_get_bootprop(char *prop);
-extern void spa_free_bootprop(char *prop);
-
-#ifdef	__cplusplus
-}
-#endif
-
-#endif	/* _SYS_SPA_BOOT_H */