Remove reference to deleted md4.c

Reviewed-by: Kurt Roeckx <kurt@openssl.org>
diff --git a/crypto/md4/Makefile b/crypto/md4/Makefile
index 11866d7..e32ef0e 100644
--- a/crypto/md4/Makefile
+++ b/crypto/md4/Makefile
@@ -15,7 +15,7 @@
 
 GENERAL=Makefile
 TEST=md4test.c
-APPS=md4.c
+APPS=
 
 LIB=$(TOP)/libcrypto.a
 LIBSRC=md4_dgst.c md4_one.c