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/linux/zfs/zfs_vfsops.c

@@ -56,7 +56,6 @@
 #include <sys/sunddi.h>
 #include <sys/dmu_objset.h>
 #include <sys/dsl_dir.h>
-#include <sys/spa_boot.h>
 #include <sys/objlist.h>
 #include <sys/zpl.h>
 #include <linux/vfs_compat.h>