doc: Remove kernel-parameters.txt entry for rcutorture.read_exit

There is only ever the one read-exit task, and there is no module
parameter named rcutorture.read_exit, so remove the bogus documentation.
Instead, use rcutorture.read_exit_burst to enable/disable read-exit
race testing.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: <bpf@vger.kernel.org>
Reviewed-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

Author: paulmckrcu

Documentation/admin-guide/kernel-parameters.txt

@@ -5412,11 +5412,6 @@
 			Set time (jiffies) between CPU-hotplug operations,
 			or zero to disable CPU-hotplug testing.
 
-	rcutorture.read_exit= [KNL]
-			Set the number of read-then-exit kthreads used
-			to test the interaction of RCU updaters and
-			task-exit processing.
-
 	rcutorture.read_exit_burst= [KNL]
 			The number of times in a given read-then-exit
 			episode that a set of read-then-exit kthreads