[wallet] Delete unused account functions

Summary:
Deletes:

• ReadAccount
• WriteAccount
• EraseAccount
• DeleteLabel
• GetLabelDestination

Partial backport of Core PR13825
https://github.com/bitcoin/bitcoin/pull/13825/commits/f85988ca7440e9ac3b33250d03ff68befb9bb51c
https://github.com/bitcoin/bitcoin/pull/13825/commits/53038b47ec51ac455f448b2d324a2bc032c45d68

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/D5419