[Cashtab] Remove bignumber.js library

Summary:
The slpMdm library uses bignumber.js, and makes it available. No need for Cashtab to have bignumber.js installed separately, and sometimes use both.

Ultimately would like to deprecate bignumber.js altogether, but for now it is still worth keeping slpMdm as we do not have a better library for making to-spec slp v1 txs.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15072