BUILD: no need to clean up when making git-tar

git-tar uses the repository, not the working dir, so it's useless to
run "make clean" first.
diff --git a/Makefile b/Makefile
index 9c7a75f..fb6ce98 100644
--- a/Makefile
+++ b/Makefile
@@ -677,7 +677,7 @@
 	    -cf - haproxy-$(VERSION)/* | gzip -c9 >haproxy-$(VERSION).tar.gz
 	rm -f haproxy-$(VERSION)
 
-git-tar: clean
+git-tar:
 	git archive --format=tar --prefix="haproxy-$(VERSION)/" HEAD | gzip -9 > haproxy-$(VERSION)$(SUBVERS).tar.gz
 
 version: