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_labels.py

@@ -13,10 +13,8 @@ from collections import defaultdict
 
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import assert_equal, assert_raises_rpc_error
-from test_framework.wallet_util import (
-    labels_value,
-    test_address,
-)
+from test_framework.wallet_util import test_address
+
 
 class WalletLabelsTest(BitcoinTestFramework):
     def set_test_params(self):
@@ -157,12 +155,7 @@ class Label:
         if self.receive_address is not None:
             assert self.receive_address in self.addresses
         for address in self.addresses:
-            test_address(
-                node,
-                address,
-                label=self.name,
-                labels=labels_value(name=self.name, purpose=self.purpose[address])
-             )
+            test_address(node, address, label=self.name, labels=[self.name])
         assert self.name in node.listlabels()
         assert_equal(
             node.getaddressesbylabel(self.name),