chore: back out resources PCD does not ship (VPNaaS, Octavia L7)

[PF9-pushkar]

Summary

Live inspection of the CE lab's service catalog and Octavia provider list confirmed which built resources correspond to features PCD actually ships. Two do not, so they're removed to keep the first-party provider aligned with PCD.

VPNaaS — entire family removed

internal/services/vpnaas (5 resources + acc test), its 5 provider registrations, the 5 examples, and the vpnaas→"VPN" doc subcategory.

The Neutron vpnaas extension is advertised in the API, but PCD does not productize/support VPNaaS — so a first-party PCD provider shouldn't expose it. (Re-add if PCD ships it later; the code is in git history.)

Octavia L7 — pcd_lb_l7policy + pcd_lb_l7rule removed

PCD ships only the OVN Octavia provider — verified on the lab:

GET /octavia/v2/lbaas/providers  ->  { "providers": [ { "name": "ovn" } ] }
GET .../providers/amphora/...    ->  HTTP 400  (absent)

OVN is a pure L4 load balancer with no HTTP awareness, so L7 policies/rules can never function on PCD. The now-unused rootLBIDFromL7Policy helper and the l7policies import were removed from loadbalancer.go; splitParentChildID stays (the member resource still uses it).

The remaining five LB resources (loadbalancer/listener/pool/member/monitor) work on OVN with L4 constraints (TCP/UDP/SCTP protocols, OVN pool algorithms, L4 health monitors) — a usage note (added to the loadbalancer resource description), not a reason to remove them.

Overbuild check — nothing else

Every other family maps to a live, shipped catalog service: identity, image, network, compute, volumev3, dns, key-manager, load-balancer (all HTTP 200). PCD-internal services we (correctly) never built for: hamgr, watcher, mors, placement, regioninfo, resmgr.

Validation

• go build/vet/gofmt/golangci-lint clean (0 issues — no dead code left behind); all unit tests pass.
• make generate renders cleanly: 5 lb_* resource pages (no l7), no vpnaas pages, no template errors. terraform fmt clean.
• CHANGELOG + DECISIONS updated, including a dated backout entry and a correction to the earlier "VPNaaS is shippable" compatibility note.