arch/um/kernel/mem.c: remove arch_validate()

- Remove arch_validate(), because no one uses it.

- Remove useless macro HAVE_ARCH_VALIDATE.

- Make the variable 'empty_bad_page' static.

Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: WANG Cong <wangcong@zeuux.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>