integrations: Remove ancient legacy trello plugin docs.

This has been "legacy" since 2016, so surely nobody is using it.

templates/zerver/integrations/trello-plugin.md

@@ -1,19 +0,0 @@
-This legacy integration for Trello works by running a script that
-communicates with the Trello API in a cron job. We recommend
-[the new Trello webhook integration](./trello) (which will
-eventually replace this entirely). Please
-[report any issues](https://github.com/zulip/zulip/issues/new)
-which cause you to prefer this integration over the webhook
-integration.
-
-{!create-stream.md!}
-
-Next, download a copy of
-[trello-to-zulip](https://github.com/zulip/trello-to-zulip)
-and follow the instructions in `README.md`.
-
-{!congrats.md!}
-
-![Trello bot message](/static/images/integrations/trello/002.png)
-
-##### Thanks to Nathan from FoundationDB for submitting this integration!

zerver/lib/integrations.py

@@ -609,17 +609,6 @@ INTEGRATIONS: Dict[str, Integration] = {
     ),
     "svn": Integration("svn", "svn", ["version-control"], doc="zerver/integrations/svn.md"),
     "trac": Integration("trac", "trac", ["project-management"], doc="zerver/integrations/trac.md"),
-    "trello-plugin": Integration(
-        "trello-plugin",
-        "trello-plugin",
-        ["project-management"],
-        logo="images/integrations/logos/trello.svg",
-        secondary_line_text="(legacy)",
-        display_name="Trello",
-        doc="zerver/integrations/trello-plugin.md",
-        stream_name="trello",
-        legacy=True,
-    ),
     "twitter": Integration(
         "twitter",
         "twitter",