[Cashtab] Remove documentation link from settings

Summary:
T2841

As part of deprecating docs, remove link to it from Cashtab.

Test Plan:
npm start
Navigate to settings page
the link isn't there, and nothing else looks too messed up from the change

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12781