rpc: deprecate "warning" field in {create,load,unload,restore}wallet

This string field has been replaced in these four RPCs by a "warnings" field
returning a JSON array of strings.

test/functional/wallet_createwallet.py

@@ -25,6 +25,7 @@ class CreateWalletTest(BitcoinTestFramework):
 
     def set_test_params(self):
         self.num_nodes = 1
+        self.extra_args = [["-deprecatedrpc=walletwarningfield"]]
 
     def skip_test_if_missing_module(self):
         self.skip_if_no_wallet()