kcms/rules: Make Comboboxes bordered again

The property `flat: true`was set since the QML redesign and remained
unnoticed because it didn't actually work.

Now that the qqc2-desktop style has been fixed and correctly draws
flat (non-bordered) comboboxes let's remove it for a nicer look
(just the same as the have had all this time)

src/kcms/rules/package/contents/ui/RuleItemDelegate.qml

@@ -51,7 +51,6 @@ Kirigami.AbstractListItem {
                 Layout.preferredWidth: 50  // 50%
                 Layout.fillWidth: true
                 Layout.alignment: Qt.AlignVCenter
-                flat: true
 
                 visible: count > 0
                 enabled: ruleEnabled

src/kcms/rules/package/contents/ui/ValueEditor.qml

@@ -96,7 +96,6 @@ Loader {
     Component {
         id: optionEditor
         OptionsComboBox {
-            flat: true
             model: ruleOptions
             onActivated: (index) => {
                 valueEditor.valueEdited(currentValue);
@@ -107,7 +106,6 @@ Loader {
     Component {
         id: netTypesEditor
         OptionsComboBox {
-            flat: true
             model: ruleOptions
             multipleChoice: true
             // Filter the provided value with the options mask
@@ -122,7 +120,6 @@ Loader {
         id: optionListEditor
         OptionsComboBox {
             id: optionListCombo
-            flat: true
             model: ruleOptions
             multipleChoice: true