move to src/product/index.scss per @shawnbot's suggestion

Co-Authored-By: Shawn Allen <shawn.allen@github.com>

Author: vdepizzol

src/core/index.scss

@@ -21,7 +21,6 @@
 @import "../box/index.scss";
 @import "../breadcrumb/index.scss";
 @import "../buttons/index.scss";
-@import "../dropdowns/index.scss";
 @import "../table-object/index.scss";
 @import "../forms/index.scss";
 @import "../layout/index.scss";