[Cashtab] Deprecate handleEncryptedOpReturn

Summary: Now that D12045 has landed there is no need for the handleEncryptedOpReturn() function as this is now part of sendXec().

Test Plan:

• npm test
• grep -r 'handleEncryptedOpReturn' src/ and ensure no other uses for the function throughout cashtab
• npm start
• send and receive encrypted messages on an existing wallet
• send and receive encrypted messages on a fresh new wallet

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D12055