packaging: Do not purge cached tarballs on every packaging target invocation.

Some tarballs are expensive to generate.  It's useful to be able to
reuse those.  The cleanup is moved to the clean target.

Change-Id: Ibe2b379be60dbb4b5c4eea9f0eeae4be84ccdb16

Makefile

@@ -69,8 +69,6 @@ $(RELEASE_TARBALL_FILENAME):
 	        $(shell basename $(CURDIR)) && \
 	    mv $(RELEASE_TARBALL_FILENAME) $(CURDIR)
 
-	rm -rf $(CURDIR)/daemon/contrib/tarballs/*
-
 #######################
 ## Packaging targets ##
 #######################
@@ -121,3 +119,4 @@ clean:
 	rm -rf packages
 	rm -f Makefile.packaging.distro_targets
 	rm -f .docker-image-*
+	rm -rf daemon/contrib/tarballs/*