[Cashtab] [final countdown bch param p1] Delete getBCH function

Summary:
T2730

Now that chronik is the only backend for Cashtab and there is no need to switch between different instances of bch-api, delete the getBCH function.

Test Plan:
npm test
grep -r getBCH src/ and no output
npm start and nothing explodes

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12805