libkms: Remove libkms completely

libkms was a very early attempt at a KMS management library, that only
got as far as handling requests to create buffers. It has since been
superseded by GBM in doing this, which everyone uses, unlike libkms
which no-one uses.

Remove it from the tree to avoid any confusion.

Signed-off-by: Daniel Stone <daniels@collabora.com>