remove animation

To make tests ok.
I could use Capybara.disable_animation = true, but there was no animation to old datetimepicker, so it's more consistant like this.

app/assets/stylesheets/admin/plugins/flatpickr-customization.scss

@@ -1,4 +1,8 @@
 .flatpickr-calendar {
+    /* Disable animation */
+    &.animate.open {
+        animation: none;
+    }
     border-radius: 0;
     &.arrowBottom:after {
         border-top-color: #eceef1;