doc: pdftex works with PNGs now, remove dependency on PDF figures

* Makefile.am: pdftex seems to work fine with PNGs now. So the need to build
  PDF versions of the figures using ImageMagick convert is gone. Remove
  the dependency. Can leave the rule for now, no benefit removing it.
diff --git a/doc/Makefile.am b/doc/Makefile.am
index 30d5dd3..4caa462 100644
--- a/doc/Makefile.am
+++ b/doc/Makefile.am
@@ -40,7 +40,7 @@
 # us to have a generic automatic .pdf rule to build the figure sources
 # because it cant just work from the png's directly it seems - contrary
 # to the documentation...
-quagga.pdf: $(info_TEXINFOS) $(figures_pdf) $(quagga_TEXINFOS)
+quagga.pdf: $(info_TEXINFOS) $(quagga_TEXINFOS)
 	$(TEXI2PDF) -o "$@" $< || true
 
 quagga_TEXINFOS = appendix.texi basic.texi bgpd.texi filter.texi \