[kcms/notifications] Reduce excess spacing between related checkboxes

The KCM looks nicer this way, now that there are three logical groups in
the severity level section.

kcms/notifications/package/contents/ui/main.qml

@@ -144,10 +144,6 @@ KCM.SimpleKCM {
             }
         }
 
-        Item {
-            Kirigami.FormData.isSection: true
-        }
-
         QtControls.CheckBox {
             Kirigami.FormData.label: i18n("Normal notifications:")
             text: i18n("Always keep on top")
@@ -161,10 +157,6 @@ KCM.SimpleKCM {
             }
         }
 
-        Item {
-            Kirigami.FormData.isSection: true
-        }
-
         QtControls.CheckBox {
             Kirigami.FormData.label: i18n("Low priority notifications:")
             text: i18n("Show popup")