For unused variables reported by pyflakes, either remove the variable,

suppress the warning, or fix a bug (if the wrong variable was used).
refs #2450

Signed-off-by: Daira Hopwood <daira@jacaranda.org>

qa/rpc-tests/hardforkdetection.py

@@ -17,7 +17,7 @@ class HardForkDetectionTest(BitcoinTestFramework):
     def setup_network(self):
         self.nodes = []
         self.alert_filename = os.path.join(self.options.tmpdir, "alert.txt")
-        with open(self.alert_filename, 'w') as f:
+        with open(self.alert_filename, 'w'):
             pass  # Just open then close to create zero-length file
         self.nodes.append(start_node(0, self.options.tmpdir,
                             ["-blockversion=2", "-alertnotify=echo %s >> \"" + self.alert_filename + "\""]))