UBUNTU: SAUCE: (no-up) do not remove debian directory by 'make mrproper'

If you use only remote build servers such as launchpad, cbd, etc.
you may not care about this, but if you run 'make mrproper', the
debian directory is entirely removed because it is a generated
directory for the upstream kernel.

I do not want to lose the debian directory by accident when I am
debugging something on my local machine.

Signed-off-by: Masahiro Yamada <masahiro.yamada@canonical.com>
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>

Makefile

@@ -1489,7 +1489,7 @@ CLEAN_FILES += vmlinux.symvers modules-only.symvers \
 # Directories & files removed with 'make mrproper'
 MRPROPER_FILES += include/config include/generated          \
 		  arch/$(SRCARCH)/include/generated .objdiff \
-		  debian snap tar-install \
+		  snap tar-install \
 		  .config .config.old .version \
 		  Module.symvers \
 		  certs/signing_key.pem \