refactor(material/core): remove unused compatibility code (#28018)

Removes some density-related code that isn't being used anywhere anymore.

(cherry picked from commit 4395cdf)

Author: crisbeto

src/material/core/density/private/_compatibility.scss

@@ -3,7 +3,6 @@
 @use 'sass:math';
 @use 'sass:meta';
 @use 'palette';
-@use '../density/private/compatibility';
 
 // Whether to enable compatibility with legacy methods for accessing theme information.
 $theme-legacy-inspection-api-compatibility: true !default;
@@ -481,20 +480,7 @@ $_internals: _mat-theming-internals-do-not-access;
   $_emitted-density: map.merge($_emitted-density, ($id: $previous-density)) !global;
   $_emitted-typography: map.merge($_emitted-typography, ($id: $previous-typography)) !global;
 
-  // If duplicate default density styles would be generated for a legacy constructed theme,
-  // we adjust the density generation so that no density styles are generated by default.
-  // If no default density styles have been generated yet, we ensure that the styles
-  // are generated at root. For legacy themes our goal is to generate default density
-  // styles **once** and at root. This matches the old behavior where density styles were
-  // part of the base component styles (that did not use view encapsulation).
-  // TODO: Remove this compatibility logic when the legacy theming API is removed.
-  compatibility.$private-density-generate-at-root: private-is-legacy-constructed-theme($theme);
-  compatibility.$private-density-generate-styles: not $duplicate-legacy-density;
-
   @content;
-
-  compatibility.$private-density-generate-at-root: false;
-  compatibility.$private-density-generate-styles: true;
 }
 
 // Checks whether the given value resolves to a theme object. Theme objects are always