_sass: Remove tight-headers mixin

_sass/custom/card.scss

 .card {
   @include abstract-card();
-  @include tight-headers();
   display: flex;
   flex-direction: column;
   min-width: 0;

_sass/custom/mixins.scss

@@ -10,14 +10,3 @@
     margin-right: 0;
   }
 }
-
-@mixin tight-headers() {
-  h1,
-  h2,
-  h3,
-  h4,
-  h5,
-  h6 {
-    margin: $sp-2 0;
-  }
-}