docs: Fix htmldocs build failure

Build of HTML docs failing due to conversion of deviceiobook.tmpl in
8a8a602f and regulator.tmpl in 028f2533 to RST without removing from
DOCBOOKS in Makefile, resulting (in the case of deviceiobook) the
following error:

make[1]: *** No rule to make target 'Documentation/DocBook/deviceiobook.xml', needed by 'Documentation/DocBook/deviceiobook.aux.xml'.  Stop.
Makefile:1452: recipe for target 'htmldocs' failed
make: *** [htmldocs] Error 2

Update DOCBOOKS to reflect available books.
Signed-off-by: Martyn Welch <martyn.welch@collabora.co.uk>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/DocBook/Makefile

@@ -7,12 +7,12 @@
 # list of DOCBOOKS.
 
 DOCBOOKS := z8530book.xml  \
-	    kernel-hacking.xml kernel-locking.xml deviceiobook.xml \
+	    kernel-hacking.xml kernel-locking.xml \
 	    writing_usb_driver.xml networking.xml \
 	    kernel-api.xml filesystems.xml lsm.xml kgdb.xml \
 	    gadget.xml libata.xml mtdnand.xml librs.xml rapidio.xml \
 	    genericirq.xml s390-drivers.xml uio-howto.xml scsi.xml \
-	    sh.xml regulator.xml w1.xml \
+	    sh.xml w1.xml \
 	    writing_musb_glue_layer.xml
 
 ifeq ($(DOCBOOKS),)