drop signing API from api/v5/

[eviljeff]

Following on from a conversation in matrix, the signing API is old has limited functionality compared to the newer addon submission API, so lets drop it from api/v5. Developers can keep using the signing api under api/v4 if they need it.

┆Issue is synchronized with this Jira Task

[ioanarusiczki]

@eviljeff

The v5 endpoints on
https://addons-dev.allizom.org/api/v5/addons/ and https://addons-dev.allizom.org/api/v5/addons/{df795427-d72f-41cd-a776-33af65e743b1}/versions/7.0/ throw a 404 Not Found.

v4 and v3 can still be used -> I could create and submit new versions using them.

[KevinMind]

Old Jira Ticket: https://mozilla-hub.atlassian.net/browse/ADDSRV-316