doc: Fix RCU's docbook options

Commit 764f80798b95 ("doc: Add RCU files to docbook-generation files")
added :external: options for RCU source files in the file
Documentation/core-api/kernel-api.rst.  However, this now means nothing,
so this commit removes them.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/core-api/kernel-api.rst

@@ -352,44 +352,30 @@ Read-Copy Update (RCU)
 ----------------------
 
 .. kernel-doc:: include/linux/rcupdate.h
-   :external:
 
 .. kernel-doc:: include/linux/rcupdate_wait.h
-   :external:
 
 .. kernel-doc:: include/linux/rcutree.h
-   :external:
 
 .. kernel-doc:: kernel/rcu/tree.c
-   :external:
 
 .. kernel-doc:: kernel/rcu/tree_plugin.h
-   :external:
 
 .. kernel-doc:: kernel/rcu/tree_exp.h
-   :external:
 
 .. kernel-doc:: kernel/rcu/update.c
-   :external:
 
 .. kernel-doc:: include/linux/srcu.h
-   :external:
 
 .. kernel-doc:: kernel/rcu/srcutree.c
-   :external:
 
 .. kernel-doc:: include/linux/rculist_bl.h
-   :external:
 
 .. kernel-doc:: include/linux/rculist.h
-   :external:
 
 .. kernel-doc:: include/linux/rculist_nulls.h
-   :external:
 
 .. kernel-doc:: include/linux/rcu_sync.h
-   :external:
 
 .. kernel-doc:: kernel/rcu/sync.c
-   :external: