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