Remove unused ObligationCauseCode::ProjectionWf

compiler/rustc_middle/src/traits/mod.rs

@@ -250,9 +250,6 @@ pub enum ObligationCauseCode<'tcx> {
     /// A tuple is WF only if its middle elements are `Sized`.
     TupleElem,
 
-    /// This is the trait reference from the given projection.
-    ProjectionWf(ty::AliasTy<'tcx>),
-
     /// Must satisfy all of the where-clause predicates of the
     /// given item.
     ItemObligation(DefId),