[Cashtab] Replace address function with equivalent lib function

Summary: This was implemented in Cashtab before ecashaddrjs had this functionality. Replace Cashtab implementation with library implementation. Remove Cashtab implementation and its associated tests.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D15541