fix(material-experimental/mdc-checkbox): reduce amount of emitted theme css

We were adding the ripple variables to the MDC checkbox theme, even though we weren't actually using it. These changes remove the variables which shaves off ~30kb from the theme.

Author: crisbeto

src/material-experimental/mdc-checkbox/_checkbox-theme.scss

@@ -16,17 +16,10 @@
 @mixin private-checkbox-styles-with-color($color) {
   @include mdc-checkbox-theme.theme-deprecated(
     (
-      density-scale: null,
       checkmark-color: mdc-theme-color.prop-value(on-#{$color}),
       container-checked-color: $color,
-      container-checked-hover-color: null,
       container-disabled-color: rgba(mdc-theme-color.prop-value(on-surface), 0.38),
       outline-color: rgba(mdc-theme-color.prop-value(on-surface), 0.54),
-      outline-hover-color: null,
-      ripple-color: mdc-theme-color.prop-value(on-surface),
-      ripple-opacity: mdc-ripple-theme.$dark-ink-opacities,
-      ripple-checked-color: $color,
-      ripple-checked-opacity: mdc-ripple-theme.$dark-ink-opacities,
     )
   );
 }
@@ -71,7 +64,6 @@
     );
 
     .mat-mdc-checkbox {
-      @include private-checkbox-styles-with-color(primary);
       @include mdc-form-field.core-styles($query: mdc-helpers.$mat-theme-styles-query);
       @include ripple-theme.color((
         foreground: (