[6.x] Remove dead fluid publish-fields CSS and unused selectedWidth computed

[jasonvarga]

This clean up will be helpful in some upcoming changes regarding field widths and dashboard widget ui. #14690

• .publish-fields-fluid (publish.css) was introduced in 07202fe133 for the old
  resources/js/components/publish/Fields.vue, applied broadly across the CP. That whole
  component tree was deleted in [6.x] Remove old components #11968 in favor of ui/Publish/Fields.vue, which never
  reintroduced the class. The one screen named in the class's own comment
  (/cp/collections/pages/edit, i.e. Configure Collection) now renders via asConfig() /
  a stacked divide-y layout instead — confirmed in CollectionsController::edit(). Dead
  since 2025-07-18.

• .publish-fields-fluid--xl-only (publish.css) was added for the Blueprint Builder's
  Title/Hidden fields on 2025-09-01, then removed 5 weeks later in [6.x] Go back to v5 style config fields #12700 ("Go back to v5
  style config fields"), which replaced it with the same stacked as-config pattern. A
  deliberate UI-direction reversal, not an accidental casualty. Dead since 2025-10-09.

• .publish-fields-fluid > * (animation.css) was added alongside the Blueprint Builder
  usage above and never removed when that markup changed. A later cleanup ([6.x] Kill sibling transitions #13658) touched
  a near-duplicate selector list elsewhere in the same file but missed this one — evidence
  nobody's relied on it since, not that it's still needed. The still-live .publish-fields > *
  selector in the same rule is untouched.

• selectedWidth (resources/js/components/fields/Settings.vue) is an unused computed
  property. It's also broken dead code — it references this.widths, which isn't defined
  anywhere in that component, so it would throw if ever evaluated.

These are all internal implementation details that were never exposed as CP extension points
— no Blade view, config option, or documented customization hook ever emitted these classes,
and the computed was never referenced — so there's no addon-facing breaking-change risk.