End "custom core" primer, use regular core instead (#18887)

Author: heiskr

stylesheets/index.scss

@@ -1,35 +1,4 @@
-// Import modules needed from Primer (stored in local `node_modules` directory)
-@import "@primer/css/support/index.scss";
-
-// BEGIN CUSTOM CORE IMPORT
-// Normally we would just import @primer/css/core/index.scss, but that file
-// currently imports @primer/css/layout/index.scss, which in turn imports
-// @primer/css/layout/columns.scss, which has deprecated styles that litter
-// the logs with warnings. We're not using those styles, so the warnings are
-// avoided here by not importing the file.
-//
-// TODO: Use a simple core import when https://github.com/primer/css/issues/741 is resolved
-@import "@primer/css/base/index.scss";
-@import "@primer/css/box/index.scss";
-@import "@primer/css/buttons/index.scss";
-@import "@primer/css/forms/index.scss";
-
-// BEGIN CUSTOM LAYOUT IMPORT
-@import "@primer/css/support/index.scss";
-@import "@primer/css/layout/container.scss";
-// @import "@primer/css/layout/columns.scss"; <-- deprecated and unused
-@import "@primer/css/layout/grid.scss";
-@import "@primer/css/layout/grid-offset.scss";
-// END CUSTOM LAYOUT IMPORT
-
-@import "@primer/css/navigation/index.scss";
-@import "@primer/css/pagination/index.scss";
-@import "@primer/css/tooltips/index.scss";
-@import "@primer/css/truncate/index.scss";
-@import "@primer/css/utilities/index.scss";
-@import "@primer/css/avatars/index.scss";
-// END CUSTOM CORE IMPORT
-
+@import "@primer/css/core/index.scss";
 @import "@primer/css/markdown/index.scss";
 @import "@primer/css/forms/index.scss";
 @import "@primer/css/navigation/index.scss";
@@ -49,7 +18,6 @@ $marketing-font-path: "/dist/fonts/";
 @import "sidebar.scss";
 @import "scroll-button.scss";
 @import "explorer.scss";
-// from https://unpkg.com/highlight.js@9.15.8/styles/github.css
 @import "syntax-highlighting.scss";
 @import "tables.scss";
 @import "opacity.scss";