rpc: simplify getaddressinfo labels, deprecate previous behavior

- change the value returned in the RPC getaddressinfo `labels` field to an array
  of label name strings

- deprecate the previous behavior of returning a JSON hash structure containing
  label `name` and address `purpose` key/value pairs

- update the relevant tests

test/functional/wallet_basic.py

@@ -15,10 +15,7 @@ from test_framework.util import (
     connect_nodes,
     wait_until,
 )
-from test_framework.wallet_util import (
-    labels_value,
-    test_address,
-)
+from test_framework.wallet_util import test_address
 
 
 class WalletTest(BitcoinTestFramework):
@@ -395,7 +392,7 @@ class WalletTest(BitcoinTestFramework):
             for label in [u'рыба', u'𝅘𝅥𝅯']:
                 addr = self.nodes[0].getnewaddress()
                 self.nodes[0].setlabel(addr, label)
-                test_address(self.nodes[0], addr, label=label, labels=labels_value(name=label))
+                test_address(self.nodes[0], addr, label=label, labels=[label])
                 assert label in self.nodes[0].listlabels()
         self.nodes[0].rpc.ensure_ascii = True  # restore to default