[Cashtab] Deprecate address validator artifact

Summary:
T2730

Deprecate BCH param from EnhancedInputs.js

The address validation function using this legacy method is an artifact from Cashtab support for multiple address formats. It should have been removed a long time ago.

This diff removes it.

Test Plan:
cd web/cashtab
grep -r AddressValidators src/ and no results
npm start and address validation on Send and SendToken pages is still good

Reviewers: #bitcoin_abc, kieran709

Reviewed By: #bitcoin_abc, kieran709

Differential Revision: https://reviews.bitcoinabc.org/D12497