Update aria-rowindex mapping for IA2 to not expose via group position

[aleventhal]

Currently aria-rowindex has this mapping:

MSAA + IAccessible2	Object Attribute: rowindex:Method: IAccessible2::groupPosition(): positionInGroup= on rows

Some reasons that I think we should expose only as the rowindex object attribute, and not as the group position as well:

• It's redundant to expose rowindex in two ways.
• The object attribute "rowindex" is sufficient.
• Chrome (and I think Firefox) do not currently expose it this way, and we would like to avoid adding the complexity.
• It's strange to only expose the group position info only for rows where aria-rowindex is used (Chrome currently only exposes the group position on rows in a tree table, but then it always exposes the group position, not just when aria-rowindex is used).
• It's strange to expose the group position only in IA2, and not in other APIs.

[jcsteh]

Firefox does expose aria-rowindex in the group position. However, like Chrome, it also exposes group position without aria-rowindex.

I don't really have a strong opinion here. I agree it's redundant strictly speaking, but then, you could argue exposing the group position on rows is always redundant because of the row index on the table interface.