DPI: remove premium signatures on unregister

[gsanchietti]

A firewall without a valid subscription can't access DPI advanced signatures.

Required changes: when a firewall is unregistered, community signatures should be restored.

If there are any DPI rules using an advanced signature, the signature will not have any effect.

[github-actions[bot]]

Testing image version: 8-23.05.3-ns.0.0.5-rc2-10-gb1208cb

[andre8244]

An error occured during QA:

• Enable subscription
• Create a DPI rule and enable some apps (including one or more subscription app)
• Apply pending changes
• Disable subscription
• Go to DPI filter page: an error is show on the UI. Copying the command:

  /usr/libexec/rpcd/ns.dpi call list-rules
  Traceback (most recent call last):
  File "/usr/libexec/rpcd/ns.dpi", line 70, in <module>
  print(json.dumps({'values': dpi.list_rules(e_uci)}))
                              ^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/nethsec/dpi/__init__.py", line 291, in list_rules
  File "/usr/lib/python3.11/site-packages/nethsec/dpi/__init__.py", line 132, in __load_blocklist
  File "/usr/lib/python3.11/site-packages/nethsec/dpi/__init__.py", line 30, in __load_applications
  FileNotFoundError: [Errno 2] No such file or directory: '/etc/netify.d/netify-apps.conf'

[github-actions[bot]]

Testing image version: 8-23.05.3-ns.0.0.5-rc2-26-gb3db7a3

[andre8244]

Verified