Revert "Better errors for implied static bound"

This reverts commit c75817b0a7.

compiler/rustc_middle/src/traits/mod.rs

@@ -10,7 +10,6 @@ mod structural_impls;
 pub mod util;
 
 use crate::infer::canonical::Canonical;
-use crate::mir::ConstraintCategory;
 use crate::ty::abstract_const::NotConstEvaluatable;
 use crate::ty::subst::SubstsRef;
 use crate::ty::{self, AdtKind, Ty, TyCtxt};
@@ -184,16 +183,6 @@ impl<'tcx> ObligationCause<'tcx> {
             variant(DerivedObligationCause { parent_trait_pred, parent_code: self.code }).into();
         self
     }
-
-    pub fn to_constraint_category(&self) -> ConstraintCategory<'tcx> {
-        match self.code() {
-            MatchImpl(cause, _) => cause.to_constraint_category(),
-            AscribeUserTypeProvePredicate(predicate_span) => {
-                ConstraintCategory::Predicate(*predicate_span)
-            }
-            _ => ConstraintCategory::BoringNoLocation,
-        }
-    }
 }
 
 #[derive(Clone, Debug, PartialEq, Eq, Hash, Lift)]
@@ -429,8 +418,6 @@ pub enum ObligationCauseCode<'tcx> {
         is_lit: bool,
         output_ty: Option<Ty<'tcx>>,
     },
-
-    AscribeUserTypeProvePredicate(Span),
 }
 
 /// The 'location' at which we try to perform HIR-based wf checking.