Remove systemd-networkd from Manifest

[webern]

Once https://github.com/bottlerocket-os/bottlerocket/pull/3700 lands, there will no longer be a need for this field

https://github.com/bottlerocket-os/twoliter/blob/40e21e03aecc18de092f9cd94bb045e6c3028d8d/tools/buildsys/src/manifest.rs#L536 https://github.com/bottlerocket-os/twoliter/blob/40e21e03aecc18de092f9cd94bb045e6c3028d8d/tools/buildsys/src/manifest.rs#L201

          Can you open an issue in twoliter to remove this feature flag if we're not using it? We should cut it from `buildsys` and `Dockerfile`.

Originally posted by @bcressey in https://github.com/bottlerocket-os/bottlerocket/pull/3700#discussion_r1449605953

[bcressey]

Even after that PR is merged, we need to keep the feature around until the next minor release, just in case there's a point release in the interim.