Remove AccessibilityPrepare event.

Remove AccessibilityPrepare event.

This event was completely unused.
In addition it leads to crashes on linux when
sending the Destroy accessibility update.

The Destroy event on linux would still query an accessible interface.
That in turn would trigger the event to be sent.

Change-Id: I8915527de067b8b70ba41b1361e3ef5d12866d7d
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@nokia.com>
Reviewed-by: Jan-Arve Sæther <jan-arve.saether@nokia.com>