Deprecate wallet 'account' API

Summary:
[wallet] [rpc] Deprecate wallet 'account' API
[wallet] [tests] Add tests for accounts/labels APIs

This commit finalizes the deprecation of the wallet 'accounts' API by
removing all account arguments and return values.

RPC behaviour is slightly different if the 'accounts' or 'labels' API is
being used. Those behaviour changes are fully documented in the RPC help
text.

Partial backport of Core PR12953
https://github.com/bitcoin/bitcoin/pull/12953/commits/109e05dcd163b8ddb7f4b3550db6b9ab833b2c04
https://github.com/bitcoin/bitcoin/pull/12953/commits/72c9575f7b793e3c77bd2d631ef7345a4c31cb0d

Depends on D4369

Test Plan:

make check
test_runner.py
./bitcoind -deprecatedrpc=accounts
./bitcoin-cli help getbalance
./bitcoin-cli help sendmany
./bitcoin-cli help listtransactions
./bitcoin-cli help listsinceblock
./bitcoin-cli help getaddressinfo

Repeat the above help commands, but under ./bitcoind and verify the help messages are correct.

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: Fabien, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4370