Remove mixin 'mx_Dialog_link' (#10787)

There is not a component named 'mx_Dialog_link', and removing a mixin is good for pushing our style codebase toward vanilla CSS.

res/css/_common.pcss

@@ -857,8 +857,3 @@ legend {
         }
     }
 }
-
-@define-mixin mx_Dialog_link {
-    color: $accent;
-    text-decoration: none;
-}