[Cashtab] [Address simplification p4/4] Remove unused toLegacyToken function

Summary:
T2730

Depends on D12653

This function was only used in SendToken.js, where it was removed in part 1. So, delete the function and its associated unit tests.

Test Plan:
npm test
grep -r toLegacyToken src/ to confirm it is nowhere in Cashtab

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12654