Remove the automatic creation and loading of the default wallet
Summary:
No longer create a default wallet. The default wallet will still be
loaded if it exists and not other wallets were specified (anywhere,
including settings.json, bitcoin.conf, and command line).
Tests are updated to be started with -wallet= if they need the default
wallet.
Added test to wallet_startup.py testing that no default wallet is
created and that it is loaded if it exists and no other wallets were
specified.
Tell users how to load or create a wallet when no wallet is loaded
This is a backport of core#15454 and core#19971 (bug fix)
Depends on D10240
Test Plan: ninja all check check-functional-extended
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Subscribers: Fabien
Differential Revision: https://reviews.bitcoinabc.org/D10243