refactor: Remove undocumented experimental roughunder command

[Omikhleia]

Since Aug. 2022 (or earlier, can't remember and lazy to check), the framebox package has an experimental and undocumented \roughunder command using a sketchy line(s) for a replacement of the "underline" command. As noted in-code:

This would need to be re-implemented and checked after multiline effects (e.g. multiline links and underline) are possibly added to the typesetter.

Such "liners" spanning multiple lines are making it in SILE develop branch for 0.15 and likewise in silex.sile, which would allow to better implement this...
But the whole topic is moved to Omikhleia/resilient.sile#75 and Omikhleia/resilient.sile#76

--> Here we remove the undocumented command.