Remove orphan CSS selectors in core/z-index.css
Summary:
* `project-board-header`
** Added in 183dd2a62764f0c2b5fd5f234cc4356d657a1069, usage removed in https://we.phorge.it/rP6349741760a84de46f6a53d57fadef3b044b15ee also from `webroot/rsrc/css/phui/phui-workboard-view.css` but not from `core/z-index.css`
* `pholio-new-inline-comment`
** Added in https://we.phorge.it/rPe5af167ec99cdb29fcb3bb44e44c5afd3d0c6d7e, `.pholio-new-inline-comment` was removed from `webroot/rsrc/css/application/pholio/pholio-inline-comments.css` in https://we.phorge.it/rP85870e79aaf2f5dd0065f7b84d2e491062929027 but not from `core/z-index.css`; usage was removed in https://we.phorge.it/rP942f504893578c361c02727a4b615c70dc761af9 by deleting `src/applications/pholio/view/PholioInlineCommentSaveView.php`
For further cleanups in `core/z-index.css`, see D25997.
Test Plan: Grep the code for each CSS selector (`grep --exclude="*.css" -R 'some-search-string' .`), also do partial searches, get no results.
Reviewers: O1 Blessed Committers, valerio.bozzolan
Reviewed By: O1 Blessed Committers, valerio.bozzolan
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Differential Revision: https://we.phorge.it/D26010