[Cashtab] [ECPair from wif p3/3] Consistent camel case for ECPair

Summary:
T2730

Depends on D12618

Apply consistent capitalization to ECPair by removing the lone occuring instance of EcPair

Test Plan:
grep -r EcPair src/ and confirm no output
npm start and send tx

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12619