[native][web] Get rid of unnecessary React state for QR code URL

Summary:
This can be derived from other state. We should avoid duplicating state to prevent the possibility of it becoming inconsistent.

Depends on D12107

Test Plan: I tested the flow using staging, with my phone as a primary device and a local simulator as a secondary device

Reviewers: bartek

Reviewed By: bartek

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12108