[Cashtab] Remove obsolete docs link

Summary:
T2930

Remove obsolete docs link

Test Plan:
npm start
navigate to localhost:3000 in an incognito window
confirm docs link is not present

grep -r 'docs.cashtab.com' src/ and no output

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D13082