daemon: remove UPnP support

Keep the "-upnp" option as a hidden arg for one major version in order
to show a more user friendly error to people who had this option set in
their config file.

test/functional/test_framework/util.py

@@ -433,7 +433,6 @@ def write_config(config_path, *, n, chain, extra_config="", disable_autoconnect=
         # in tests.
         f.write("peertimeout=999999999\n")
         f.write("printtoconsole=0\n")
-        f.write("upnp=0\n")
         f.write("natpmp=0\n")
         f.write("shrinkdebugfile=0\n")
         f.write("deprecatedrpc=create_bdb\n")  # Required to run the tests