Don't overwrite the bg-variant mixin

Bootstrap 4.3 introduced an additional parameter, and npm run dev fails
because we are overwriting a mixin that bootstrap uses internally.

resources/assets/sass/core/mixins/_background-variant.scss

@@ -1,16 +1,3 @@
-// Contextual backgrounds
-@mixin bg-variant($parent, $color) {
-    #{$parent} {
-        background-color: $color !important;
-    }
-    a#{$parent},
-    button#{$parent} {
-        @include hover-focus {
-            background-color: darken($color, 10%) !important;
-        }
-    }
-}
-
 @mixin bg-gradient-variant($parent, $color) {
     #{$parent} {
         background: linear-gradient(87deg, $color 0, adjust-hue($color, 25%) 100%) !important;