lightningd: don't simply ignore defaults on flags, deprecate.

Changelog-Deprecated: Plugins: `default` no longer accepted on `flag` type parameters (it was silently ignored, so just don't set it).
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

contrib/pyln-client/pyln/client/plugin.py

@@ -912,7 +912,7 @@ class Plugin(object):
                 m["long_description"] = method.long_desc
 
         manifest = {
-            'options': list(self.options.values()),
+            'options': list({k: v for k, v in d.items() if v is not None} for d in self.options.values()),
             'rpcmethods': methods,
             'subscriptions': list(self.subscriptions.keys()),
             'hooks': hooks,