DocBook: Cleanup: remove an unused $(call) line

There's no build_images function to call. So remove it.

This is just a cleanup patch, with doesn't affect the build.
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/DocBook/Makefile

@@ -51,7 +51,6 @@ pdfdocs: $(PDF)
 HTML := $(sort $(patsubst %.xml, %.html, $(BOOKS)))
 htmldocs: $(HTML)
 	$(call build_main_index)
-	$(call build_images)
 	$(call install_media_images)
 
 MAN := $(patsubst %.xml, %.9, $(BOOKS))