[@vercel/blob] Deprecate useCache option on get() to a no-op (#1075)

The backend no longer honors the cache=0 query parameter that useCache: false
produced, so the option had no effect. Gut the implementation (reads always go
through the standard caching path) and remove its tests. The typed option is
kept and marked @deprecated so existing callers compile without error; it will
be removed in a future major version.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>