[Cashtab] Remove artifact CashtabSettings type from formatting

Summary:
This was defined in the formatting file before CashtabSettings was converted into a class.

It's no longer used here or elsewhere. Type is now defined in src/config/CashtabSettings.ts

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17446