10 Planned Comic Book Movie Scenes We’ll Never Get To See
6. The X-Men Prison Break

According to writer Simon Kinberg, Rogue was also removed because her involvement meant that Kitty Pryde's injury wouldn't have mattered: she could have absorbed her powers and continued in her place, and that would have taken away some of the threat of the future sequence. That meant that the prison break sequence had to be dropped entirely - all ten minutes or so of it, though Brian Singer did admit that it was a good scene:
Through the editing process, the sequence became extraneous. Its a really good sequence and it will probably end up on the DVD so people can see it. But like many things in the editing process, it was an embarrassment of riches and it was just one of the things that had to go."
Hopefully the entire sequence was filmed in its entirety, because it sounds like it could have been incredible.