[wallet] Remove strFromAccount and strSentAccount

Summary:
No longer used.

Partial backport of Core PR13825
https://github.com/bitcoin/bitcoin/pull/13825/commits/1031fa0c53f8f19ba27c951979fdfb64f64e70c4
https://github.com/bitcoin/bitcoin/pull/13825/commits/87e8123539f392985dd5778b335d71999b4e2f29

Depends on D5316

Test Plan:

ninja check
test_runner.py wallet_*

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5421