Remove WindowedValue.getPane() in preparation for making WindowedValue a user-facing interface

[kennknowles]

This is a revival of #35136 which has already been reviewed but I closed it in order to address issues in a phased manner.

Less important that the public interface, this change also helps the interface work better with tools like AutoBuilder that benefit from very uniform naming, instead of sometimes being called paneInfo and sometimes being called pane.

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