layout: Refactor inline layout to remove the code that tried to avoid splitting fragments.

I don't think it will be possible to avoid splitting fragments in the
presence of `vertical-align`, because one `ScannedTextFragment` could
potentially be split into arbitrary many fragments, each having its own
vertical position that can influence layout of other fragments.

This code also removes parts of `Range` that were no longer used.

r? @glennw
Should these TODOs be retained?