[Cashtab] Disable rewards in the extension

Summary:
Browser extension security requirements make implementing recaptcha-v3 difficult. Looking around, it might be possible, but it requires manually saving some remote code and modifying the security policy.

The rewards are great but since they work on the web app, I don't think including them in the extension is worth this implementation and maintenance headache. Since they do not work now, we should not show them as available.

Test Plan:
npm run extension and there is no Rewards option in the menu. Create a new wallet and there is no reward claim option on its home screen.

npm start and rewards screen is available (tho rewards are not expected to work from localhost:3000)

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17220