uml: remove unused header

From: Adrian Bunk <bunk@kernel.org>

This patch removes the unused and broken (the normal asm/keyboard.h
files no longer exists) include/asm-um/keyboard.h

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Cc: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>