Remove webui from Saturn L2 node

[bajtos]

L2-node is bundling filecoin-saturn/node-webui and serving it over HTTP, so that people running L2 nodes (e.g. inside Station) can access the dashboard.

This was a very early implementation of Saturn GUI for the Station. The Station was rendering node-webui served by L2-node inside an iframe. A lot has changed since then, the Station no longer renders any webui from L2-node.

I think we can safely remove node-webui from L2-node:

• It was a quick hack to begin with.
• If we want L2 nodes to show webui, then we should start with UX research to find out what kind of dashboard makes the most sense for L2 operators. It is likely to be different from a dashboard for L1 operators.

Our developer docs for the Station hasn’t been updated after we stopped rendering Saturn webui, so it illustrates well the original architecture: https://github.com/filecoin-station/filecoin-station/blob/main/docs/DEVELOPING.md

We don't have a production need for the L2 single node stats webui currently. Once L2s provide value outside of Station, people might consider running L2s in headless mode, and then might want to see the stats UI.

Tasks

• Remove all webui-related code from Saturn L2 node. In particular:
  • The script downloading webui into L2-node: https://github.com/filecoin-saturn/L2-node/blob/main/scripts/download-webui.sh
  • The code implementing /webui handler in L2-node: https://github.com/filecoin-saturn/L2-node/blob/ae78fe055f49a9c97eb099bf1ccb74e2e23c0ef7/cmd/saturn-l2/main.go#L303-L305
• Update our Developer Guide to describe the new architecture

[bajtos]

Opened #1239 to update our Developer Guide.

I think nobody cares about Saturn's L2 node anymore. Closing this issue as not planned.