Don't mirror isAccessibilityElement on _ASTableViewCell

[bdolman]

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.