lenovo/*: Remove unused MAINBOARD_VERSION

Change-Id: I88e889efe43b32e7efc59ea40d13e79209a27264
Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Reviewed-on: http://review.coreboot.org/7179
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins)