fix: #3612540 [11.4 regression] Fix Toolbar with Default Admin

Jürgen Haasrequested to merge
issue/drupal-3612540:3612540-11.4-regression-fix into main

Fix broken toolbar styling in the experimental Default Admin theme.

The theme's libraries-override for toolbar/toolbar remaps the toolbar module's structural stylesheet (css/toolbar.module.css) to a theme file css/components/toolbar.module.css that does not ship with the theme (and has no .pcss.css source). The browser therefore requests a non-existent file, receives the 404 HTML error page, and refuses it:

Refused to apply style from '.../default_admin/css/components/toolbar.module.css' because its MIME type ('text/html') is not a supported stylesheet MIME type.

This strips the toolbar's structural CSS (#toolbar-administration resets, box-sizing, layout), which breaks Admin Toolbar rendering.

Remove the bogus component override so the toolbar module's own css/toolbar.module.css loads normally. The theme's theme-level overrides (toolbar.theme.css / toolbar.icons.theme.css), which do exist, are kept.

Closes #3612540

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