Don't use graphic icons for notes, warnings, etc.

Change 30758 [1] fixed a bug with showing graphics in the tutorial,
but also included the icons folder which attempted to provide
graphics for notes, warnings, and other asciidoc sections.  These
graphics are not currently in our repository and there are licensing
concerns with using them.  To simplify things, we are not going to
include graphics for these icons and will keep the previous 2.2.1
behavior.

[1] https://gerrit-review.googlesource.com/#/c/30785/

Change-Id: Ic4132122d8011f4e590d15d5323c0b645835f604
Signed-off-by: Brad Larson <bklarson@gmail.com>

Documentation/Makefile

@@ -75,7 +75,6 @@ $(DOC_HTML): %.html : %.txt
 	@rm -f $@+ $@
 	@$(ASCIIDOC) -a toc \
 		-a data-uri \
-		-a icons \
 		-a 'revision=$(REVISION)' \
 		-b xhtml11 \
 		-f asciidoc.conf \