[LoopUnrollAndJam] Forget scalar evolution dispositions. Do no explicitly forget subloop.

Fixes https://github.com/llvm/llvm-project/issues/58454

Scalar evolution dispositions need to be forgotten to pass verification.

We do not need to forget the subloop since it is automatically forgotten
when forgetting the parent loop.

Differential Revision: https://reviews.llvm.org/D140953