include/linux/kdev_t.h: remove unused huge_valid_dev()

There's no user of huge_valid_dev() any more, so remove it.

No functional change.

Signed-off-by: Yaowei Bai <bywxiaobai@163.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>