[Cashtab] Deprecate BCH param from sendXec

Summary:
T2730

Depends on D12667

Now that BCH has been deprecated from all functions in sendXec, deprecate from sendXec

This has cascading requirements that can't be split out into a separate diff on the stack as unit tests will fail. jestBCH and the BCH object must be removed from Send.js as they are no longer used. They are also removed from associated snapshot tests.

Test Plan:
npm start
Send a one-to-one XEC tx
Send a one-to-many XEC tx
grep -r sendXec src/ and confirm the BCH param is removed from all instances

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12668