The Delete operation doesn't properly call
TextEditorAction.validateEditorInputState() and as a result doesn't work
in version control environments that keep unchanged files read-only.
Another problem is that Ctrl+Z doesn't trigger Undo when the Outline
view is active. When the Delete element action is reintroduced it should
be made to behave the same way as in JDT, e.g, display a prompt before
deletion and be bound to the Delete key.
This reverts commit f034a69fb7.
Change-Id: I6d37f5aaea1770143fbf84288068794430a5b0e0
6335a5c2ba