refactor: Combine GetWalletForJSONRPCRequest and EnsureWalletIsAvailable functions

Summary:
This simplifies control flow and also helps get rid of the ::vpwallets
variable, because EnsureWalletIsAvailable doesn't have access to the request
context.

This is a backport of core#19100

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9452