Get rid of `applied_iname_rewrites`

Suggested by @kaushikcfd in https://github.com/inducer/loopy/pull/631#discussion_r907887383.

The original thinking behind it was transform usability. Suppose one splits an iname. The one thing that this currently enables is the use of the old iname in `add_prefetch`. That's less awkward than having to manually specify the split iname expression, but I do agree that this is kind of broken as a concept, and definitely not strong enough justification to drag this around in the IR.

Usage site for reference: https://github.com/inducer/loopy/blob/7d2f7d1eb43e0ebf923e141f68879ea0e10d9ac9/loopy/transform/data.py#L98-L101=