remove diverging type variables from fn check

The comment seems incorrect. Testing revealed that the examples in
question still work (as well as some variants) even without the
special casing here.

compiler/rustc_infer/src/infer/type_variable.rs

@@ -129,7 +129,11 @@ pub enum TypeVariableOriginKind {
     SubstitutionPlaceholder,
     AutoDeref,
     AdjustmentType,
-    DivergingFn,
+
+    /// In type check, when we are type checking a function that
+    /// returns `-> dyn Foo`, we substitute a type variable for the
+    /// return type for diagnostic purposes.
+    DynReturnFn,
     LatticeVariable,
 }