buildsys: rework clean target, avoid doc-gen one

1. this really doesn't change often
2. the synopsis and opts should be in the owner repo anyway
3. the original one simply deleted all *.adoc files, far to
   aggressive

Avoids pve-docs dependency for building the DSC (without having to
pass the ugly no-pre-clean option).

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

src/Makefile

@@ -50,5 +50,4 @@ test:
 .PHONY: clean distclean
 distclean: clean
 clean:
-	make cleanup-docgen
-	find . -name '*~' -exec rm {} ';'
+	rm -f *.xml.tmp *.1 *.5 *.8 *{synopsis,opts}.adoc docinfo.xml