Remove Tailscale feature from devcontainer.json

[dguido]

Problem

devcontainer.json includes Tailscale as a feature:

"features": {
    "ghcr.io/devcontainers/features/github-cli:1": {},
    "ghcr.io/tailscale/codespace/tailscale:1": {}
}

Per @DarkaMaul's review on trailofbits/skills#26, Tailscale is not generic enough for a base template. Users who need it can add it themselves.

Already removed in trailofbits/skills devcontainer-setup plugin.