Don't mirror isAccessibilityElement on _ASTableViewCell

This is a partial revert of #1941

The change in #1941 has caused some issues for us and others (see #1997).
If I set `myNode.isAccessibilityElement = true,` then what ends up
happening is that the table cell wrapper view (_ASTableViewCell) also
has `isAccessibilityElement = true`. But then when iOS goes through the
hierarchy it identifies the table cell wrapper view as the target and
attempts to use that accessibilityLabel, which doesn't exist
(because it's actually on the nested node view).
So our node's label is never used.

I'm not sure why setting `isAccessibilityElement` in `_ASTableViewCell`
was necessary in #1941 in order to make a node non-accessible,
since the default value is already false.

Author: bdolman

Source/ASTableView.mm

@@ -116,7 +116,6 @@ - (void)setElement:(ASCollectionElement *)element
     self.selectionStyle = node.selectionStyle; 
     self.focusStyle = node.focusStyle;
     self.accessoryType = node.accessoryType;
-    self.isAccessibilityElement = node.isAccessibilityElement;
     self.accessibilityElementsHidden = node.accessibilityElementsHidden;
     // the following ensures that we clip the entire cell to it's bounds if node.clipsToBounds is set (the default)
     // This is actually a workaround for a bug we are seeing in some rare cases (selected background view