This was added for a Hetzner IPv6 migration
(756b8fe0b5) and could be useful again,
but it rebuilds `params.json` via `Vm#params_json` from scratch just
to patch a small subset of fields.
I’m about to add more required keyword arguments to `params_json`,
which will exacerbate this brittleness.
I considered fixes but wanted to delay deciding between two options,
hence deleting for now:
1. Refactor network field generation into a helper method shared by
both `params_json` and `UpdateIpv6`, keeping the representation
coupled and well-exercised.
2. Persist all data needed for `params_json` regeneration. Currently
only `swap_size_bytes` is transient (passed this way), but we’re
adding more parameters soon.
We can restore and rework this code if needed again.
0b66dd0478