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 \