Remove WARNS?= 6 from Makefile, since it is already in src/bin/Makefile.inc

Requested by:	ru
Index: src/bin/mkdir/Makefile
===================================================================
--- src/bin/mkdir/Makefile	(revision 153)
+++ src/bin/mkdir/Makefile	(working copy)
@@ -2,6 +2,5 @@
 # $FreeBSD: src/bin/mkdir/Makefile,v 1.7 2001/12/04 01:57:45 obrien Exp $
 
 PROG=	mkdir
-WARNS?=	6
 
 .include <bsd.prog.mk>