selinux: remove unnecessary assignment to subdir-

Makefile.clean descends into $(subdir-y).  Dummy assignment to subdir-
is meaningless.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Paul Moore <paul@paul-moore.com>

scripts/selinux/Makefile

 subdir-y := mdp genheaders
-subdir-	+= mdp genheaders