MMU2 improved front PTFE holder

Improved front PTFE holder to be used with this improved selector:
https://www.thingiverse.com/thing:4219114

I modified the Prusa R2 holder to have openings under the bolt holes to prevent failures caused by cut of pieces of filament getting trapped in the holes. Before replacing my front ptfe holder with this design I had several failed loads caused by the cutter cutting a small piece of filament and that piece dropped in one of the bolt holes and jamming the selector.