She could just be back for the time travel part and any emotional stuff with Iceman and Kitty. But it's a war and she is likely escaping mutant prison so for her not to have some kind of fight scene would be highly unrealistic. I don't think she'd get close enough to a Sentinel to do any damage. They are almost certainly mutant killers so Kitty's powers aren't the most useful when they'd blast her away before she could get close enough to phase through them. Her and Professor X's powers are probably the least useful against Sentinels. All the others can fight from a bit further away or aren't as easy to blast. I could be wrong. I am just not expecting Kitty to have a big bang scene like that. Though she may have an action scene of her escaping a Sentinel attack. The temple looks like it gets blown apart.