Remove the WG private keys from the vault and store them only on the servers

With the support for network.wireguard.* credentials[1] in systemd
v256[2], we can now easily avoid storing the credentials centrally in
our ansible vault, which is preferable as it makes the private keys less
exposed. It may also make fine-grained access easier in the future[3] as
there is no longer a vault file for each server.

All the keys have been rotated and the new private keys are only stored
on the servers.

[1] https://github.com/systemd/systemd/pull/30826
[2] https://github.com/systemd/systemd/releases/tag/v256
[3] #64