scripted-diff: Remove setup_clean_chain if default is not changed

-BEGIN VERIFY SCRIPT-
git grep -l "self.setup_clean_chain = False" test/functional/*.py | xargs sed -i "/self.setup_clean_chain = False/d";
-END VERIFY SCRIPT-

test/functional/p2p_blocksonly.py

@@ -15,7 +15,6 @@ from test_framework.util import assert_equal
 
 class P2PBlocksOnly(BitcoinTestFramework):
     def set_test_params(self):
-        self.setup_clean_chain = False
         self.num_nodes = 1
         self.extra_args = [["-blocksonly"]]