displaySelectedCompletion now does nothing if the autocompleteTextView is hidden.

This method is intended to display the completion selected in the table view. Prior to this commit, it proceeded even if the text view was hidden, only checking so to determine if it should insert text into the superTextView. Now it simply does not proceed with any insertions if the autocompleteTextView is hidden.
Removed deleteCompletion method call upon pressing escape.

When the user presses escape and the autocomplete view is shown, it sends the setDisplayAutocompletions:NO withAnimation:YES message, which now deletes the completion. Therefore, it does not need to be used in the textView:doCommandBySelector: method as well.