style: Switch text decoration computation over to a derived property

`-servo-text-decorations-in-effect`.

This mirrors what WebKit does; WebKit's property is called
`-webkit-text-decorations-in-effect`. It allows us to simplify a lot of
layout code--most notably, the `FlowFlagsInfo` is gone. This commit is a
necessary predecessor to removal of `InlineParentInfo`, which itself is
a necessary predecessor to reenabling parallel layout.

r? @SimonSapin