wallet: Make -wallet setting not create wallets

This changes -wallet setting to only load existing wallets, not create new ones.

- Fixes settings.json corner cases reported by sjors & promag:
  https://github.com/bitcoin-core/gui/issues/95,
  https://github.com/bitcoin/bitcoin/pull/19754#issuecomment-685858578,
  https://github.com/bitcoin/bitcoin/pull/19754#issuecomment-685858578

- Prevents accidental creation of wallets reported most recently by jb55
  http://www.erisian.com.au/bitcoin-core-dev/log-2020-09-14.html#l-355

- Simplifies behavior after #15454. #15454 took the big step of disabling
  creation of the default wallet. This PR extends it to avoid creating other
  wallets as well. With this change, new wallets just aren't created on
  startup, instead of sometimes being created, sometimes not. #15454 release
  notes are updated here and are simpler.

This change should be targeted for 0.21.0. It's a bug fix and simplifies
behavior of the #15937 / #19754 / #15454 features added in 0.21.0.

doc/release-notes.md

@@ -292,15 +292,18 @@ Wallet
   changed from `-32601` (method not found) to `-18` (wallet not found).
   (#20101)
 
-### Default Wallet
+### Automatic wallet creation removed
 
-Bitcoin Core will no longer create an unnamed `""` wallet by default when no
-wallet is specified on the command line or in the configuration files. For
-backwards compatibility, if an unnamed `""` wallet already exists and would
-have been loaded previously, then it will still be loaded. Users without an
-unnamed `""` wallet and without any other wallets to be loaded on startup will
-be prompted to either choose a wallet to load, or to create a new wallet.
-(#15454)
+Bitcoin Core will no longer automatically create new wallets on startup. It will
+load existing wallets specified by `-wallet` options on the command line or in
+`bitcoin.conf` or `settings.json` files. And by default it will also load a
+top-level unnamed ("") wallet. However, if specified wallets don't exist,
+Bitcoin Core will now just log warnings instead of creating new wallets with
+new keys and addresses like previous releases did.
+
+New wallets can be created through the GUI (which has a more prominent create
+wallet option), through the `bitcoin-cli createwallet` or `bitcoin-wallet
+create` commands, or the `createwallet` RPC. (#15454)
 
 ### Experimental Descriptor Wallets