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: NRandy Dunlap <rdunlap@infradead.org>
Reported-by: NAkira Yokosawa <akiyks@gmail.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>