Remove WalletLocation class

This removes a source of complexity and indirection that makes it harder to
understand path checking code. Path checks will be simplified in upcoming
commits.

There is no change in behavior in this commit other than a slightly more
descriptive error message in `loadwallet` if the default "" wallet can't be
found. (The error message is improved more in upcoming commit "wallet: Remove
path checking code from loadwallet RPC".)

src/wallet/test/scriptpubkeyman_tests.cpp

@@ -19,7 +19,7 @@ BOOST_AUTO_TEST_CASE(CanProvide)
     // Set up wallet and keyman variables.
     NodeContext node;
     std::unique_ptr<interfaces::Chain> chain = interfaces::MakeChain(node);
-    CWallet wallet(chain.get(), WalletLocation(), CreateDummyWalletDatabase());
+    CWallet wallet(chain.get(), "", CreateDummyWalletDatabase());
     LegacyScriptPubKeyMan& keyman = *wallet.GetOrCreateLegacyScriptPubKeyMan();
 
     // Make a 1 of 2 multisig script