do not run optipng anymore

* doc/Makefile.am (dist-hook): Remove.
* HACKING: Adjust.

HACKING

@@ -46,8 +46,6 @@ only for certain operations (like releases):
 
   pandoc    used during Debian packaging for the conversion of
             IPython notebooks to html
-  optipng   used during "make dist" if present, to optimize
-            distributed png images
   R         used by some example in the documentation (the
             documentation will still compile without R, but that
             example will appear broken)

doc/Makefile.am

@@ -147,10 +147,4 @@ EXTRA_DIST = \
   $(srcdir)/org-stamp \
   $(srcdir)/userdoc
 
-
-dist-hook: optipng
-.PHONY: optipng
-optipng:
-	-(optipng -v >/dev/null) && optipng -o5 $(distdir)/userdoc/*.png
-
 DISTCLEANFILES = org/plantuml.jar