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

module/zfs/spa.c

@@ -81,7 +81,6 @@
 #include <sys/arc.h>
 #include <sys/callb.h>
 #include <sys/systeminfo.h>
-#include <sys/spa_boot.h>
 #include <sys/zfs_ioctl.h>
 #include <sys/dsl_scan.h>
 #include <sys/zfeature.h>

module/zfs/spa_boot.c

@@ -1,50 +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.
- */
-
-#ifdef _KERNEL
-
-#include <sys/zio.h>
-#include <sys/spa_boot.h>
-#include <sys/sunddi.h>
-
-char *
-spa_get_bootprop(char *propname)
-{
-	char *value;
-
-	if (ddi_prop_lookup_string(DDI_DEV_T_ANY, ddi_root_node(),
-	    DDI_PROP_DONTPASS, propname, &value) != DDI_SUCCESS)
-		return (NULL);
-	return (value);
-}
-
-void
-spa_free_bootprop(char *value)
-{
-	ddi_prop_free(value);
-}
-
-#endif /* _KERNEL */