[Cashtab] [service worker fix p1] remove cache-first service worker

Summary:
T2684

Part 1 of a stacked diff to patch issue where Cashtab's 'cache first' service worker will not deliver the latest version of Cashtab without a hard refresh.

In this part, the existing service worker is removed.

Test Plan:
npm start
Open the dev console. Go to the Application tab and click "Service Workers." Observe no service workers in the pane.
Do the same at Cashtab.com. Observe there is a service worker.

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12020