Switch to the online version of the LaTeX2e unofficial reference manual

The tarme.rb script complains about case-insensitive conflicting files:

src/data/help/unofficial-latex2e-reference-manual/color-models-RGB.html src/data/help/unofficial-latex2e-reference-manual/color-models-rgb.html

To solve this and ease maintenance in the future, we should try to switch to the online version of the LaTeX2e unofficial reference manual (and remove the offline copy from Kile).