[ML] Ignore mouse events on pseudo element. (#20668)

The pseudo-element agg-type-select-wrapper::after which represents the dropdown's arrow didn't react to the click element of the dropdown itself. pointer-events: none is now added to the pseudo-elements to let the click pass through to the element intented to react on the click.

x-pack/plugins/ml/public/jobs/new_job/simple/components/fields_selection/styles/main.less

@@ -57,6 +57,7 @@
           border-left: 4px solid transparent;
           right: 10px;
           top: 14px;
+          pointer-events: none;
         }
       }
     }

x-pack/plugins/ml/public/jobs/new_job/simple/components/fields_selection_population/styles/main.less

@@ -63,6 +63,7 @@
             border-left: 4px solid transparent;
             right: 43px;
             top: 14px;
+            pointer-events: none;
           }
         }