s390: remove pointless drivers-y in drivers/s390/Makefile

This is unused.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

drivers/s390/Makefile

@@ -4,6 +4,3 @@
 #
 
 obj-y += cio/ block/ char/ crypto/ net/ scsi/ virtio/
-
-drivers-y += drivers/s390/built-in.a
-