[Cashtab] [Remove BCH param from wallet derivation p3/3] Delete unused mocks

Summary:
T2836

Depends on D12750

Delete unused mocks

Test Plan:
grep -r legacyAlphaTestnet src/ and no result
grep -r migratedLegacyAlphaTestnet src/ and no result

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12751