um: Remove meaningless clearing of clean-files

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/kernel/Makefile

@@ -13,7 +13,6 @@ CPPFLAGS_vmlinux.lds := -DSTART=$(LDS_START)		\
                         -DELF_FORMAT=$(LDS_ELF_FORMAT)	\
 			$(LDS_EXTRA)
 extra-y := vmlinux.lds
-clean-files :=
 
 obj-y = config.o exec.o exitcode.o irq.o ksyms.o mem.o \
 	physmem.o process.o ptrace.o reboot.o sigio.o \