Remove abstract BaseWallet

[jamshale]

We have switched to only using the AskarWallet wallet class and have no plans on supporting another wallet type. We should remove the abstract BaseWallet. This will help simplify the codebase and also helps from a developer perspective for ease of navigating the codebase into the wallet functionality.

[ff137]

My 2c worth here is that adding wallet abstraction again later would obviously be much harder. Even though it's not foreseeable to support anything besides askar now, that might change in the future.

Unless there is a conscious decision that acapy will only ever run with askar, I think the wallet abstraction might be worth keeping, just to have the scaffolding.