Remove out-of-date comment.

The type and variant mentioned no longer exist, so I suspect this is no
longer relevant.

Author: nnethercote

compiler/rustc_infer/src/infer/mod.rs

@@ -527,9 +527,6 @@ pub enum RegionVariableOrigin {
     Coercion(Span),
 
     /// Region variables created as the values for early-bound regions.
-    ///
-    /// FIXME(@lcnr): This can also store a `DefId`, similar to
-    /// `TypeVariableOriginKind::TypeParameterDefinition`.
     RegionParameterDefinition(Span, Symbol),
 
     /// Region variables created when instantiating a binder with