Remove FIXME since there is nothing to be fixed.

The errors are deduplicated when displayed to users. They only appear
multiple times in UI tests.

src/test/rustdoc-ui/ambiguous-inherent-assoc-ty.rs

@@ -11,7 +11,6 @@ impl Struct {
     pub const AssocConst: Self::AssocTy = 42;
     //~^ ERROR ambiguous associated type
     //~| HELP use fully-qualified syntax
-    // FIXME: for some reason, the error is shown twice with rustdoc but only once with rustc
     //~| ERROR ambiguous associated type
     //~| HELP use fully-qualified syntax
 }