docs: don't require 'dia' to build the docs

These diagrams are not expected to change much, so just include the generated
PNGs in the repository, and remove 'dia' from the prerequisites to build the
documentation.

docs/reference/api/Makefile.am

@@ -1,24 +1,16 @@
 
-# Generation of the additional PNG files from DIA files
-#  (list of original '.dia' files with '.png' extension)
-DIAGRAMS = \
-	ModemManager-states.dia \
-	ModemManager-interface-initialization-sequence.dia \
-	ModemManager-interface-initialization-sequence-subclassed.dia
-
-# The wanted PNG files
-DIAGRAMS_PNG = $(notdir $(DIAGRAMS:.dia=.png))
-
-# DIA to PNG conversion
-$(DIAGRAMS_PNG): $(DIAGRAMS)
-	$(AM_V_GEN) $(DIA) -t png $^ 1>/dev/null 2>&1
-
 # Logos
 LOGOS_PNG = \
 	$(top_srcdir)/data/ModemManager-logo-square.png \
 	$(top_srcdir)/data/ModemManager-logo-wide.png \
 	$(top_srcdir)/data/ModemManager-logo-wide-text.png
 
+# Diagrams
+DIAGRAMS_PNG = \
+	$(top_srcdir)/data/ModemManager-states.png \
+	$(top_srcdir)/data/ModemManager-interface-initialization-sequence.png \
+	$(top_srcdir)/data/ModemManager-interface-initialization-sequence-subclassed.png
+
 # The name of the module.
 DOC_MODULE = ModemManager
 
@@ -122,8 +114,7 @@ CLEANFILES += \
 	$(NULL)
 
 # PNGs are removed only in dist-clean
-# Diagrams are generated by dia in the current directory
-# Logos are copied to current directory by gtk-doc
+# Diagrams and logos are generated by dia in the current directory
 DISTCLEANFILES = \
-	$(DIAGRAMS_PNG) \
+	$(notdir $(DIAGRAMS_PNG)) \
 	$(notdir $(LOGOS_PNG))