Only generate transclusion contents for clipboard

Parsoid doesn't care about the transclusion contents and it can
cause bugs if not removed properly. Remove the about group attribute
as that was for Parsoid's benefit.

Depends on Ib29972472d in core.

Bug: 60897
Change-Id: I907b2113c257baff5c022ff0749dca9dbb1fba5f
Merge "Only generate transclusion contents for clipboard"