MM-11905: delete plugin commands on removal (#9601)

* defer plugin tear down for testing

* test expected plugin command unregistration

* MM-11905: uninstall plugin commands on remove

app/plugin_install.go

@@ -120,6 +120,8 @@ func (a *App) removePlugin(id string) *model.AppError {
 		a.Publish(message)
 	}
 
+	a.UnregisterPluginCommands(id)
+
 	a.Plugins.Deactivate(id)
 
 	err = os.RemoveAll(pluginPath)