NNS1-2905: Remove transactions fields from accounts (step 2)

[dskloetd]

DO NOT MERGE

This PR should only be merged after we have a release on mainnet containing https://github.com/dfinity/nns-dapp/pull/4800.

Motivation

This is the follow-up to https://github.com/dfinity/nns-dapp/pull/4800. In the previous PR we removed the transactions fields but still encoded to stable memory including the fields. In this PR we stop encoding the old fields completely. There will still be accounts in stable memory with the fields but they will be ignored when reading and removed when writing. We have not yet decided if we also want to do a migration round to remove all the fields from stable memory.

Changes

1. Stop converting Account to OldAccount before encoding it to stable memory.

Tests

1. upgrade-downgrade-test passed.
2. Manually tested upgrading and downgrading while creating new subaccounts before, in between and after.

Todos

• [x] Add entry to changelog (if necessary).