MAGETWO-81426: Magento UI - Duplicated mixins parameters #11276

 - Merge Pull Request #11276 from Igloczek/magento2:patch-2
 - Merged commits:
   1. b401d62

Author: Volodymyr Kublytskyi

lib/web/css/source/lib/_dropdowns.less

@@ -86,7 +86,6 @@
         @_dropdown-list-border,
         @_dropdown-list-z-index,
         @_dropdown-list-shadow,
-        @_dropdown-list-background,
         @_dropdown-list-item-padding,
         @_dropdown-list-item-margin,
         @_dropdown-list-item-border,
@@ -196,7 +195,6 @@
         @_dropdown-split-list-border,
         @_dropdown-split-list-z-index,
         @_dropdown-split-list-shadow,
-        @_dropdown-split-list-background,
         @_dropdown-split-list-item-padding,
         @_dropdown-split-list-item-margin,
         @_dropdown-split-list-item-border,
@@ -227,7 +225,6 @@
     @_dropdown-list-border,
     @_dropdown-list-z-index,
     @_dropdown-list-shadow,
-    @_dropdown-list-background,
     @_dropdown-list-item-padding,
     @_dropdown-list-item-margin,
     @_dropdown-list-item-border,