include/linux/kdev_t.h: remove new_valid_dev()

As all new_valid_dev() checks have been removed it's time to drop
new_valid_dev() itself.

No functional change.

Signed-off-by: Yaowei Bai <baiyaowei@cmss.chinamobile.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>