[Cashtab] Remove legacy css

Summary: A good deal of css in App.css is not relevant to the Cashtab app. This diff removes css not relevant to present Cashtab functionality. It also adds comments for dev reference on how to refactor remaining css in future diffs.

Test Plan: npm start ; confirm app still looks ok.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9299