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/test_framework/wallet_util.py

@@ -88,11 +88,6 @@ def get_multisig(node):
                     p2sh_p2wsh_script=CScript([OP_HASH160, witness_script, OP_EQUAL]).hex(),
                     p2sh_p2wsh_addr=script_to_p2sh_p2wsh(script_code))
 
-def labels_value(name="", purpose="receive"):
-    """Generate a getaddressinfo labels array from a name and purpose.
-    Often used as the value of a labels kwarg for calling test_address below."""
-    return [{"name": name, "purpose": purpose}]
-
 def test_address(node, address, **kwargs):
     """Get address info for `address` and test whether the returned values are as expected."""
     addr_info = node.getaddressinfo(address)