Remove `useConvex`

[ethan-huo]

convex/react has already exported useConvex, and re‑exporting this function in convex‑react‑query will cause errors in the latest version combination. In convex‑react‑query, useConvex() will return undefined.

[patrikduksin]

+1 any function from @convex-dev/react-query cannot find convex client in the context