[Cashtab] Deprecate BCH from getMintAddress

Summary:
T2730

Depends on D12669

Deprecate the BCH param from the getMintAddress function

As this means BCH is no longer used in Airdrop.js, requires deprecating from there as well in this diff.

Test Plan:
npm test
npm start
Calculate an airdrop tx with minting address ignored

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12670