[backport#16227 7/8] Remove CCryptoKeyStore and move all of it's functionality into CWallet

Summary:
Instead of having a separate CCryptoKeyStore that handles the encryption
stuff, just roll it all into CWallet.

https://github.com/bitcoin/bitcoin/pull/16227/commits/8f5b81e6edae9cb22559545de63f391d97c15701

Depends on D6558

Partial backport of Core PR16227

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6560