Remove systemd-networkd from Manifest

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_

Start by checking whether Bottlerocket pull request 3700 has landed, then inspect tools/buildsys/src/manifest.rs at the referenced lines and the Dockerfile for uses of the systemd-networkd feature flag. Remove the unused flag from both locations and verify that no remaining manifest or build references depend on it.

Written by the indexing model from the issue text.