doc: Remove obsolete kernel-per-CPU-kthreads RCU_FAST_NO_HZ advice

This document advises building with both CONFIG_NO_HZ=y and
CONFIG_RCU_FAST_NO_HZ=y.  However, CONFIG_NO_HZ=y offloads callbacks from
all nohz_full CPUs, and CPUs with offloaded callbacks do not benefit from
CONFIG_RCU_FAST_NO_HZ=y.  Quite the opposite: CONFIG_RCU_FAST_NO_HZ=y
simply adds a bit of idle entry/exit overhead.

This commit therefore changes that advice to only CONFIG_NO_HZ=y.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Documentation/admin-guide/kernel-per-CPU-kthreads.rst

@@ -208,7 +208,7 @@ Do at least one of the following:
 2.	Enable RCU to do its processing remotely via dyntick-idle by
 	doing all of the following:
 
-	a.	Build with CONFIG_NO_HZ=y and CONFIG_RCU_FAST_NO_HZ=y.
+	a.	Build with CONFIG_NO_HZ=y.
 	b.	Ensure that the CPU goes idle frequently, allowing other
 		CPUs to detect that it has passed through an RCU quiescent
 		state.	If the kernel is built with CONFIG_NO_HZ_FULL=y,