Remove dead nginx-tailing remote_syslog2 from setup-common.sh (non-web nodes)

## Background

`setup-common.sh` installs `remote_syslog2` with a `/etc/log_files.yml` that tails `/var/log/nginx/*.err`:

https://github.com/compiler-explorer/infra/blob/main/setup-common.sh#L80-L107 (around the `remote_syslog`/`log_files.yml`/`remote-syslog.service` block)

`setup-common.sh` is sourced by seven node types:

- `setup-node.sh` (web -- **does** run nginx, so valid here)
- `setup-aarch64-node.sh`
- `setup-ci.sh`
- `setup-admin.sh`
- `setup-builder.sh`
- `setup-ce-router.sh`
- `setup-smb.sh`

Only the web `node` runs nginx. On the other six, `remote_syslog2` runs and tails `/var/log/nginx/*.err`, a file that never exists -- so the daemon does nothing useful there. (rsyslog/grafana-agent cover real logging.)

## Why separate from #2158

The conan box has its own hand-copied duplicate of this block and was cleaned up in #2158 (conan does not source `setup-common.sh`, so it's self-contained). Editing `setup-common.sh` re-bakes all seven AMIs above, which is a much wider blast radius and deserves its own change + rebake plan.

## Proposed work

- Decide per node type whether any log-file tailing is actually wanted (only the web `node` plausibly needs nginx tailing).
- Either gate the `remote_syslog2` block behind a per-node flag, or remove it from `setup-common.sh` and add it back only where genuinely needed.
- Also bump the pinned `remote_syslog2 v0.21` (≈2020) if the daemon is kept anywhere.
- Re-bake affected AMIs.

Found while debugging the 2026-06-04 conan outage (#2158).