stubdom: no need to clean mini-os

All objects are placed inside stubdom's directories, so there is no need
to enter mini-os and clean.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Samuel Thibault <samuel.thibault@ens-lyon.org>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

stubdom/Makefile

@@ -504,7 +504,6 @@ clean:
 	rm -fr mini-os-$(XEN_TARGET_ARCH)-xenstore
 	rm -fr mini-os-$(XEN_TARGET_ARCH)-vtpm
 	rm -fr mini-os-$(XEN_TARGET_ARCH)-vtpmmgr
-	$(MAKE) DESTDIR= -C $(MINI_OS) clean
 	$(MAKE) DESTDIR= -C caml clean
 	$(MAKE) DESTDIR= -C c clean
 	$(MAKE) -C vtpm clean