[Passes] Remove legacy LoopUnswitch pass.

The legacy LoopUnswitch pass is only used in the legacy pass manager
pipeline, which is deprecated.

The NewPM replacement is SimpleLoopUnswitch and I think it is time to
remove the legacy LoopUnswitch code.

Fixes #31000.

Reviewed By: aeubanks, Meinersbur, asbirlea

Differential Revision: https://reviews.llvm.org/D124376