[wallet] Remove CAccount and Update zapwallettxes comment

Summary:
No longer used

Partial backport of Core PR13825
https://github.com/bitcoin/bitcoin/pull/13825/commits/a92bdd174956491457ac162af910573e59c350c7
https://github.com/bitcoin/bitcoin/pull/13825/commits/30538846964e8c7490f94d0b57580e65aaf9c5a9

Depends on D5419

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/D5422