[qa] Remove hardcoded "4 nodes" from test_framework

qa/rpc-tests/mempool_reorg.py

@@ -13,6 +13,10 @@ from test_framework.util import *
 
 # Create one-input, one-output, no-fee transaction:
 class MempoolCoinbaseTest(BitcoinTestFramework):
+    def __init__(self):
+        super().__init__()
+        self.num_nodes = 2
+        self.setup_clean_chain = False
 
     alert_filename = None  # Set by setup_network