rcutorture: Remove obsolete BOOTPARAM_HOTPLUG_CPU0 Kconfig option

Now that the BOOTPARAM_HOTPLUG_CPU0 Kconfig option is in the process of
being removed, it is time to remove rcutorture's use of it.

Link: https://lore.kernel.org/lkml/20230414232309.510911744@linutronix.de/Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: <x86@kernel.org>

tools/testing/selftests/rcutorture/configs/rcu/TREE01

@@ -15,4 +15,3 @@ CONFIG_DEBUG_LOCK_ALLOC=n
 CONFIG_RCU_BOOST=n
 CONFIG_DEBUG_OBJECTS_RCU_HEAD=n
 CONFIG_RCU_EXPERT=y
-CONFIG_BOOTPARAM_HOTPLUG_CPU0=y