test: remove unused class NodePongAdd1

This class was introduced in commit
fa33654 ("net: Use mockable time for
ping/pong, add tests"), but actually never used.

test/functional/p2p_ping.py

@@ -12,27 +12,21 @@
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import assert_equal
 
+
 PING_INTERVAL = 2 * 60
+TIMEOUT_INTERVAL = 20 * 60
 
 
 class msg_pong_corrupt(msg_pong):
     def serialize(self):
         return b""
 
 
-class NodePongAdd1(P2PInterface):
-    def on_ping(self, message):
-        self.send_message(msg_pong(message.nonce + 1))
-
-
 class NodeNoPong(P2PInterface):
     def on_ping(self, message):
         pass
 
 
-TIMEOUT_INTERVAL = 20 * 60
-
-
 class PingPongTest(BitcoinTestFramework):
     def set_test_params(self):
         self.setup_clean_chain = True