fix(material/theming): Expose the theme-remove Sass function (#28314)

We intended to expose this function along with the other theme
inspection APIs, but forgot to add it to the list. It is necessary for
users to be able to create their own versions of mixins like
`all-component-colors`

Author: mmalerba

src/material/_index.scss

@@ -151,4 +151,4 @@
 
 // New theming APIs:
 @forward './core/theming/inspection' show get-theme-version, get-theme-type, get-theme-color,
-  get-theme-typography, get-theme-density, theme-has;
+  get-theme-typography, get-theme-density, theme-has, theme-remove;