Remove legacy <7.15 client navigation fallback from react-router

[chargome]

With the minimum React Router version raised to 7.15, the client instrumentation API hooks always fire, so the legacy instrumentHydratedRouter navigation fallback is dead code.

Remove the !isClientInstrumentationApiUsed() navigate-patching fallback in client/hydratedRouter.ts (and simplify the surrounding logic that only existed to support React Router < 7.15).

Part of #22290.

[linear-code]

JS-3126

[chargome]

leaving as is as it provides a good best effort fallback for users that do not use the intrumentations api