Remove FIXME comment

Signed-off-by: Zahari Dichev <zaharidichev@gmail.com>

tests/test/builtin_impls/unsize.rs

@@ -68,9 +68,7 @@ fn dyn_to_dyn_unsizing() {
             "Unique; substitution [], lifetime constraints [InEnvironment { environment: Env([]), goal: '!1_0: '!1_0 }]"
         }
 
-        // Different order of traits in target and source
-        // FIXME: this doesn't work because trait object unification
-        // respects where clause order, which it shouldn't
+        // Different order of traits in target and sourcex
         goal {
             forall<'a> {
                 dyn Principal + Auto1 + 'a: Unsize<dyn Auto1 + Principal + 'a>