fix(mixins): removed multilined comment

- some node versions with `node-sass` on windows can't compile it into 1 line

Closes #9002

Author: EladBezalel

src/core/style/mixins.scss

@@ -285,10 +285,8 @@
   }
 }
 
-/**
- * Mixin to create a primary checkbox.
- * Used by the checkbox and select component.
- */
+// Mixin to create a primary checkbox.
+// Used by the checkbox and select component.
 @mixin checkbox-primary($checkedSelector: '.md-checked') {
   .md-ripple {
     color: '{{primary-600}}';