linux_chromiumos_3_18: remove kernel due lack of maintainer/breakage

There is no maintainer for this package, probably not many users.
It requires effort to fix all third-party modules for this old kernel
versions. It might contain unpatched security holes.

For Pixel chromebooks, we have the samus-kernel.
Apart from that https://github.com/GalliumOS/linux might be a good choice.