docs: don't try to get comments from rcupdate_wait.h or rcutree.h

Neither file contains any kerneldoc comments, so including them generates
these warnings in the docs build:

  ./include/linux/rcupdate_wait.h:1: warning: no structured comments found
  ./include/linux/rcutree.h:1: warning: no structured comments found

Remove them and make life a little quieter.
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/core-api/kernel-api.rst

@@ -356,10 +356,6 @@ Read-Copy Update (RCU)
 
 .. kernel-doc:: include/linux/rcupdate.h
 
-.. kernel-doc:: include/linux/rcupdate_wait.h
-
-.. kernel-doc:: include/linux/rcutree.h
-
 .. kernel-doc:: kernel/rcu/tree.c
 
 .. kernel-doc:: kernel/rcu/tree_plugin.h