rcuscale: Ditch rcu_scale_shutdown in favor of torture_shutdown_init()

The torture_shutdown_init() function spawns a shutdown kthread in
a manner very similar to that implemented by rcu_scale_shutdown().
This commit therefore re-implements rcu_scale_shutdown() in terms of
torture_shutdown_init().

This patch was generated by Claude given as input the patch making the
same transformation of ref_scale_shutdown().

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>