[Cashtab] remove unused qr code border css

Summary: Remove unused css

Test Plan:

cd web/cashtab/src
grep -nr 'react-qrcode-logo' .

Verify that there are no components with this ID in the app

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9307