Remove legacy /version.json dependency from nd httpapi

[mtarking]

Summary

The unified Nexus Dashboard (4.2+) no longer exposes the legacy /version.json endpoint. This PR removes the dead code path that depended on it, so nothing in the collection can accidentally query a non-existent endpoint on the new controller.

What changed

• plugins/httpapi/nd.py — removed the get_version(platform="nd") branch that fetched /version.json. The NDFC path (get_version("ndfc"), hardcoded 12) that cisco.dcnm modules rely on is unchanged.
• plugins/module_utils/nd.py — removed the now-unused NDModule.version property (and its self._version init), which was the only caller of get_version("nd").
• Added a changelog fragment.

Why it's safe

• get_version("nd") / NDModule.version was only consumed by the nd_version module (removed for 2.0.0 in the companion PR). Grep confirmed no surviving module reads .version — the manage/interface/fabric modules go through the state machine + orchestrators + REST sender, not this property.
• cisco.dcnm calls get_version() with the default "ndfc", so it is unaffected.

Verification done locally

• python -m py_compile passes for both edited files.
• Changelog fragment YAML validated.
• Grep confirms no remaining /version.json or get_version("nd") references; only the legitimate NDFC self.version shim remains.

Scope note

This is the httpapi cleanup split out from the legacy-module-removal PR to keep each diff focused. It does not add a runtime generation guard (get_platform_generation()); that can follow separately if/when modules need to gate on ND generation.