remove wallet birthday height from onboarding flow

[TalDerei]

wallet birthday's should be abstracted as an internal implementation detail that's not exposed in the UI, in line with the philosophy that seed phrases are the sole information needed to completely recover all wallet state. This additional step introduces a new pattern for key management that add unnecessary complexity to the onboarding flow. However, new wallets will still benefit from the sync efficiency gains by skipping trial decryption during the initial onboarding process.

This will be consumed as part of a more comprehensive issue focused on reworking the onboarding flow. cc @grod220