Rollup merge of #69059 - ljedrz:unused_stuff, r=Dylan-DPC

Remove a few unused objects

As far as I can tell, these won't be missed:

- `infer::region_constraints::ConstraintInfo`
- `driver::DefaultCallbacks`
- ~~`hir::intravisit::ParDeepVisitor`~~

src/librustc/infer/region_constraints/mod.rs

@@ -339,8 +339,6 @@ impl TaintDirections {
     }
 }
 
-pub struct ConstraintInfo {}
-
 impl<'tcx> RegionConstraintCollector<'tcx> {
     pub fn new() -> Self {
         Self::default()

src/librustc_driver/lib.rs

@@ -120,10 +120,6 @@ pub trait Callbacks {
     }
 }
 
-pub struct DefaultCallbacks;
-
-impl Callbacks for DefaultCallbacks {}
-
 #[derive(Default)]
 pub struct TimePassesCallbacks {
     time_passes: bool,