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/Kbuild.in

@@ -345,7 +345,6 @@ ZFS_OBJS := \
 	sha256.o \
 	skein_zfs.o \
 	spa.o \
-	spa_boot.o \
 	spa_checkpoint.o \
 	spa_config.o \
 	spa_errlog.o \