remove some unused objects

Author: ljedrz

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

@@ -123,10 +123,6 @@ pub trait Callbacks {
     }
 }
 
-pub struct DefaultCallbacks;
-
-impl Callbacks for DefaultCallbacks {}
-
 #[derive(Default)]
 pub struct TimePassesCallbacks {
     time_passes: bool,