openapi3: remove StringMap and its pre-OriginTree origin-stripping machinery

[reuvenharrison]

The OriginTree rewrite (getkin#1142) moved origin tracking to a separate pass that extracts __origin__ before JSON conversion, so no unmarshaler sees it anymore. A whole family of StringMap / origin-stripping machinery was left behind. This removes it end to end.

Commits

1. Remove origin-stripping leftovers — three dead delete(map, originKey) calls (Encoding, Webhooks, Server.Variables), the now-unused originKey const, and the stale StringMap doc comment. (TestOrigin_* already assert __origin__ never appears as a map key.)
2. Simplify the StringMap decode — the two-step json -> map[string]any -> deepCast -> V (sorted keys) existed to strip __origin__ and control ordering; post-rewrite it equals a plain json.Unmarshal. Drop deepCast — also removes a float64 round-trip precision risk on large integers.
3. Remove the StringMap type — its only custom behavior is gone, so its two fields become plain maps: Discriminator.Mapping -> map[string]MappingRef, OAuthFlow.Scopes -> map[string]string.
4. Drop the helpers and their pass-through UnmarshalJSON — unmarshalStringMap/unmarshalStringMapP are now trivial json.Unmarshal wrappers, and the 13 named map types that called them from a custom UnmarshalJSON were pure pass-throughs. Remove those methods so the types decode via default map unmarshalling (identical result), and delete the helpers (none of the types have a MarshalJSON, so marshalling was already default).
5. Regenerate API docs (docs.sh) and add the changelog entry.

Notes

• Breaking API change: removes the exported openapi3.StringMap[V] type. The one known downstream consumer (oasdiff) needs a one-line signature change (mappingToStringMap -> map[string]openapi3.MappingRef), which is backward-compatible with the current release. Recorded in the README changelog.
• No behavior change; full openapi3 suite green, and oasdiff's full suite passes when wired to this branch via a local replace.
• Independent of openapi3: record scalar-valued map keys for origin location lookup #19 (scalar-valued map key origins): the two branches auto-merge cleanly, and openapi3: record scalar-valued map keys for origin location lookup #19's reflection-based scalar-map handling is unaffected by the field-type changes here.