lenovo: Remove duplicate devicetree.cb eventc entry.

Keep only the last one: it was the one which was really used.

Change-Id: I19132f6224d6847e615e3c582aaa6e66b0d56c7a
Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Reviewed-on: http://review.coreboot.org/7677
Tested-by: build bot (Jenkins)
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>