Remove a filter callback which always returned true. The first `!ann.id`
condition is already part of the `store.unsavedAnnotations` logic. The
second `isHighlight(...)` check did not work properly because the input
was an annotation stub object with only `id` and `$tag` fields instead
of the `Annotation` that `isHighlight` expects. For annotation stubs
with no id, it always returned false.

From a functional perspective the `isHighlight` check was also
unnecessary. `store.unsavedAnnotations()` only returns IDs of
annotations which have active drafts, but for highlights no draft is
created.