FreeBSD: remove support for FreeBSD < 13.0-RELEASE (#16372)

This includes the last 12.x release (now EOL) and 13.0 development
versions (<1300139).

Sponsored-by: https://despairlabs.com/sponsor/

Signed-off-by: Rob Norris <robn@despairlabs.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>

include/os/freebsd/spl/sys/proc.h

@@ -88,9 +88,6 @@ do_thread_create(caddr_t stk, size_t stksize, void (*proc)(void *), void *arg,
 		thread_lock(td);
 		sched_prio(td, pri);
 		sched_add(td, SRQ_BORING);
-#if __FreeBSD_version < 1300068
-		thread_unlock(td);
-#endif
 	}
 	return (td);
 }