fi-0170: Cannot both box a type and mark it optional {:#fi-0170}

A FIDL type may be either wrapped in box<T>, or suffixed with the :optional constraint, but not both:

{% include “docs/reference/fidl/language/error-catalog/label/_bad.md” %}

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="tools/fidl/fidlc/tests/fidl/bad/fi-0170.test.fidl" exclude_regexp="\/\/ (Copyright 20|Use of|found in).*" %}

Remove the :optional suffix on the type being boxed to fix this problem:

{% include “docs/reference/fidl/language/error-catalog/label/_good.md” %}

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="tools/fidl/fidlc/tests/fidl/good/fi-0170.test.fidl" exclude_regexp="\/\/ (Copyright 20|Use of|found in).*" %}