remove `Region: Deref<Target = RegionKind>` impl

[lcnr]

call .kind() in cases where it is currently used.

This impl is a relic of back when we still interned regions as &'tcx RegionKind and differs from Ty and ty::Const

[xizheyin]

How difficult is it to do this? I would love to do this.

[lcnr]

Some familiarity with the compiler helps, but it should be doable.

[xizheyin]

I can try and may need some advice if necessary.

@rustbot claim