Merge pull request #3965 from BertrandBordage/admin-dropdown

Fixes admin dropdown arrow.

wagtail/wagtailadmin/static_src/wagtailadmin/scss/components/_forms.scss

@@ -117,7 +117,6 @@ select,
         font-size: 3em;
         pointer-events: none;
         color: $color-grey-3;
-        background-color: $color-fieldset-hover;
         margin: 0 1px 1px 0;
 
         .ie & {