doc: Fix RCU's docbook options

Commit 764f8079

 ("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>