test: reduce num_nodes in feature_nulldummy.py to 1

With bitcoin#19937 (backported as ae7e4cb), GBT doesn't require peers
if using test chains, we can drop the extra node.

Author: kwvg

test/functional/feature_nulldummy.py

@@ -43,15 +43,14 @@ def invalidate_nulldummy_tx(tx):
 class NULLDUMMYTest(BitcoinTestFramework):
 
     def set_test_params(self):
-        # Need two nodes so GBT (getblocktemplate) doesn't complain that it's not connected.
-        self.num_nodes = 2
+        self.num_nodes = 1
         self.setup_clean_chain = True
         self.extra_args = [[
             '-whitelist=127.0.0.1',
             '-dip3params=105:105',
             f'-testactivationheight=bip147@{COINBASE_MATURITY + 5}',
             '-par=1',  # Use only one script thread to get the exact reject reason for testing
-        ]] * 2
+        ]]
 
     def skip_test_if_missing_module(self):
         self.skip_if_no_wallet()