Fixing a memory leak in SynEvents.js

This pull request fixes a memory leak with IE8 in `SynEvents`.
It is sufficient for `aria.utils.SynEvents` to be loaded for the leak to happen.
The issue can be reproduced with attester. As soon as several tests in the test suite extend (directly or indirectly) `aria.jsunit.TemplateTestCase`, because this class depends on `aria.utils.SynEvents`, the leak is visible.
When `SynEvents` is loaded, this class performs some tests on the browser to know how it behaves. For this purpose some DOM elements are created and some event listeners are attached to them but those listeners were never removed.
This pull request removes the leak by removing event listeners which are no longer needed.