Task lists are now properly formatted, with no bullet.
In addition, we have removed an expensive generic traverse to
remove Space elements, and replaced it with code in
`inlinesToOpenXML`. This should give better performance; it
also reduces XML size in the metadata, which wasn't previously
affected by the de-Spacing.
TODO: parse this in the reader so that we can have task lists round-trip.
Closes#5198.
