Remove outdated comment

compiler/rustc_const_eval/src/interpret/place.rs

@@ -906,10 +906,6 @@ pub fn copy_op_transmute(
         }
         // We still require the sizes to match.
         if src.layout.size != dest.layout.size {
-            // FIXME: This should be an assert instead of an error, but if we transmute within an
-            // array length computation, `typeck` may not have yet been run and errored out. In fact
-            // most likely we *are* running `typeck` right now. Investigate whether we can bail out
-            // on `typeck_results().has_errors` at all const eval entry points.
             span_bug!(
                 self.cur_span(),
                 "size-changing transmute, should have been caught by transmute checking: {:#?}\ndest: {:#?}",