wallet: Remove path checking code from createwallet RPC

This commit does not change behavior except for error messages which now
include more complete information.

test/functional/wallet_multiwallet.py

@@ -273,7 +273,8 @@ class MultiWalletTest(BitcoinTestFramework):
         self.log.info("Test dynamic wallet creation.")
 
         # Fail to create a wallet if it already exists.
-        assert_raises_rpc_error(-4, "Wallet w2 already exists.", self.nodes[0].createwallet, 'w2')
+        path = os.path.join(self.options.tmpdir, "node0", "regtest", "wallets", "w2")
+        assert_raises_rpc_error(-4, "Failed to create database path '{}'. Database already exists.".format(path), self.nodes[0].createwallet, 'w2')
 
         # Successfully create a wallet with a new name
         loadwallet_name = self.nodes[0].createwallet('w9')