sso-and-identity: remove Firefly III from native-OIDC table, verified wrong live

Live-queried Authentik's API directly: firefly (and emby, not in this
doc's table anyway) are both Proxy Provider only, no native OIDC/OAuth2
provider exists for either. Real count is 10 apps with native OIDC,
not the 11/12 both this doc and the KB previously implied. Same gap
existed in the Authentik/SSO Integration KB note, fixed there too.

docs/sso-and-identity.md

@@ -20,7 +20,6 @@ Every app gets the Proxy Provider registration described above — that's the un
 | Grafana | [`stacks/platform/grafana-stack/`](../stacks/platform/grafana-stack/) |
 | Audiobookshelf | [`stacks/media/audiobookshelf/`](../stacks/media/audiobookshelf/) |
 | Dockhand | [`stacks/platform/dockhand/`](../stacks/platform/dockhand/) |
-| Firefly III | [`stacks/productivity/firefly-iii/`](../stacks/productivity/firefly-iii/) |
 | Seerr | [`stacks/media/seerr/`](../stacks/media/seerr/) |
 | Proxmox / PBS | hypervisor-level, not a docker stack — PBS has both layers, Proxmox has native OIDC only |