test: remove legacy wallet functional tests

Removes all legacy wallet specific functional tests.

Also removes the --descriptor and --legacy-wallet options as these are
no longer necessary with the legacy wallet removed.

test/functional/rpc_help.py

@@ -43,12 +43,10 @@ def process_mapping(fname):
 
 
 class HelpRpcTest(BitcoinTestFramework):
-    def add_options(self, parser):
-        self.add_wallet_options(parser)
-
     def set_test_params(self):
         self.num_nodes = 1
         self.supports_cli = False
+        self.uses_wallet = None
 
     def run_test(self):
         self.test_client_conversion_table()