Fix errcheck error in plugin_test.go by properly handling error from th.App.ch.RemovePlugin (#30608)

server/.golangci.yml

@@ -103,7 +103,6 @@ issues:
         channels/app/platform/session.go|\
         channels/app/platform/status.go|\
         channels/app/platform/web_hub_test.go|\
-        channels/app/plugin_test.go|\
         channels/app/post_helpers_test.go|\
         channels/app/post_test.go|\
         channels/app/slack.go|\