[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