I was wondering if one of you knows any way to play Rocksmith 2014 in Ultrawide (21:9) in Fullscreen without the black Pillar Boxes left and right. 

I just made an account to +1 my interest in this. I have a 5120 x 2160 monitor that has significant pillarboxing when playing 2014 Remastered. [[runs some numbers really quick ...]] In fact - 25% of my screen isn't used! What a waste!

Officially, Ubisoft has no built-in way to adapt to anything other than a 16:9 ratio:

If you choose a resolution that’s not in the widescreen (16:9) aspect ratio, Rocksmith 2014 will letterbox [sic] the image in order to maintain its presentation proportions. The game will fill the unused window areas with black bars and only display in a subsection of your screen.

Another game I found this pillarboxing problematic was the original Hades game (they may have since patched a solution) where someone uploaded universal patcher on git. I have found no such fix for RS 2014 Remastered, but would LOVE to see one.