[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