Remove checkpoints

The headers presync logic should be enough to prevent memory DoS using
low-work headers. Therefore, we no longer have any use for checkpoints.

test/functional/test_runner.py

@@ -365,7 +365,6 @@ BASE_SCRIPTS = [
     'feature_filelock.py',
     'feature_loadblock.py',
     'wallet_assumeutxo.py --descriptors',
-    'p2p_dos_header_tree.py',
     'p2p_add_connections.py',
     'feature_bind_port_discover.py',
     'p2p_unrequested_blocks.py',