As per title I want to remove the tvdb dvd order agent, as it's now superfluous and I like to keep things tidy.
I deleted the plugin itself, I deleted its folder from the plugin Data directory, I've deleted all plugin caches, and in a moment of desperation I've also deleted everything in the "metadata combination" folder – and restarted Plex, of course. I've checked, double-checked, and triple-checked that my TV library is now set to use "the tvdb" agent and not the dvd order one (the dvd order one no longer appears in the options).
However, when I scan my library, it doesn't find metadata for new files, and my logs are full of errors like these:
Aug 06, 2017 19:45:24.712 [0x809cef800] DEBUG - HTTP 200 response from GET http://127.0.0.1:32400/system/agents/update?mediaType=2&force=1&guid=com%2Eplexapp%2Eagents%2Ethetvdbdvdorder%3A%2F%2F84912%3Flang%3Den&id=11 Aug 06, 2017 19:45:24.712 [0x809cef800] ERROR - Error response for update: 5 (Unhandled exception).
So it's pretty clear that some part of Plex, somewhere, is still clinging on to thetvdbdvdorder plugin. How do I exorcise this demon?
I've put the plugin back to stop the errors. Looking at the logs, some shows are using thetvdb and some are using thetvdbdvdorder, I'm not sure if there's logic to it.
I have found a way to make a specific TV show to use the thetvdb agent if it's using the dvd one: On the ... drop-down, choose "Unmatch", then do "Match" to set it to the right show and it will use the right agent (note that "Fix Match" doesn't change it).