[Cashtab] Remove url checker to complete migration to cashtab.com

Summary:
Initially, plan was to maintain cashtab.com and cashtabapp.com indefinitely. Local storage works by domain, so any user who has a wallet at cashtabapp.com would need to manually move to cashtab.com.

However, maintaining multiple URLs for a wallet that stores mnemonics based on domain has other downsides for security (people might expect to use cashtab at domains other than cashtab.com, which could open them up to fraud, entering seeds in the wrong website).

So, I just set up domain forwarding for cashtabapp.com to cashtab.com. Notice has been up for a couple of months now. If support requests come in, it would be possible to put the app back up at cashtabapp.com to support recovery of previous wallets.

Test Plan: npm start and observe no pop-up even though app is loading at localhost:3000 and not cashtab.com

Reviewers: emack, #bitcoin_abc

Reviewed By: emack, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10675