Drop breakpoint CSS custom properties (#29020)

Media queries do not support CSS custom properties (CSS variables), so we've had some useless variables lying around since shipping v4. This finally removes them and the associated documentation content that explained the situation.

Fixes #29012.

scss/_root.scss

@@ -8,10 +8,6 @@
     --#{$color}: #{$value};
   }
 
-  @each $bp, $value in $grid-breakpoints {
-    --breakpoint-#{$bp}: #{$value};
-  }
-
   // Use `inspect` for lists so that quoted items keep the quotes.
   // See https://github.com/sass/sass/issues/2383#issuecomment-336349172
   --font-family-sans-serif: #{inspect($font-family-sans-serif)};