[Cashtab] [Deprecate toLegacyCash p3/3] Remove now-unused functions, unit tests, and mocks

Summary:
T2730

Depends on D12656

Delete now unused functions, unit tests, and mocks

Test Plan:
grep -r toLegacyCash src/ no output
grep -r toLegacyCashArray src/ no output
Confirm removed mocks were only used in the deleted unit tests
npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12657