removed redundant and clean code less

Author: Tu Nguyen

app/design/adminhtml/Magento/backend/Magento_ConfigurableProduct/web/css/source/module/components/_currency-addon.less

@@ -18,15 +18,10 @@
 //  _____________________________________________
 
 .currency-addon {
+    .lib-vendor-prefix-display(inline-flex);
     border: 1px solid rgb(173,173,173);
-    position: relative;
-    display: -webkit-inline-flex;
-    display: -ms-inline-flexbox;
-    display: inline-flex;
-    -webkit-flex-direction: row;
-    -ms-flex-direction: row;
-    flex-direction: row;
     flex-flow: row nowrap;
+    position: relative;
     width: 100%;
 
     .admin__control-text {