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/os/freebsd/zfs/spa_os.c

@@ -58,7 +58,6 @@
 #include <sys/fs/zfs.h>
 #include <sys/arc.h>
 #include <sys/callb.h>
-#include <sys/spa_boot.h>
 #include <sys/zfs_ioctl.h>
 #include <sys/dsl_scan.h>
 #include <sys/dmu_send.h>

module/os/freebsd/zfs/zfs_vfsops.c

@@ -62,7 +62,6 @@
 #include <sys/sunddi.h>
 #include <sys/dmu_objset.h>
 #include <sys/dsl_dir.h>
-#include <sys/spa_boot.h>
 #include <sys/jail.h>
 #include <ufs/ufs/quota.h>
 #include <sys/zfs_quota.h>