[landing] Disable RainbowKit slide-in animation

Summary: Our BottomSheet is already sliding in; we don't need to do it twice.

Test Plan: Load http://localhost:3000/commlanding/siwe in desktop browser, Mobile Safari on Simulator, and within WebView in Comm app to verify slide-in effect is gone

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6010