MM-47612: Skip flaky test TestPluginAPIDeleteUserPreferences (#21425)

https://mattermost.atlassian.net/browse/MM-47612

```release-note
NONE
```

app/plugin_api_test.go

@@ -196,6 +196,7 @@ func TestPluginAPIGetUserPreferences(t *testing.T) {
 }
 
 func TestPluginAPIDeleteUserPreferences(t *testing.T) {
+	t.Skip("MM-47612")
 	th := Setup(t)
 	defer th.TearDown()
 	api := th.SetupPluginAPI()