'make clean' on website doesn't remove html files that have no parent wml file

$(HTMLFILES) in Makefile.common consists of all the $2.html.$1 files that have a corresponding $1/$2.wml file.

So when 'make clean' does "rm -f $(HTMLFILES) $(DEPFILES)" it misses foo.html.bar files whose wml file we have deleted.

For example, download/download.html.de exists and doesn't get cleaned, because download/de/ has no download.wml file.