Fixes a memory leak in startup page.

There only seems to be one global StartupPageEditorResolverContribution instance. Registering disposables to the class adds them to that instance, but being a global instance instance, the disposables are never disposed.

Before

When opening and closing a getting started editor, the number GettingStartedInputs grows each time:

After

No more leaks are detected

{
  "namedFunctionCount3": [],
  "isLeak": false
}