[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