Remove secret backup feature flag

There isn't really a point in hiding this behind a flag and it just makes the process even more confusing

src/app/neochatconfig.kcfg

@@ -211,10 +211,6 @@
       <label>Enable threads</label>
       <default>false</default>
     </entry>
-    <entry name="SecretBackup" type="bool">
-      <label>Enable secret backup</label>
-      <default>false</default>
-    </entry>
     <entry name="Phone3PId" type="bool">
       <label>Enable add phone numbers as 3PIDs</label>
       <default>false</default>

src/app/qml/AccountMenu.qml

@@ -79,7 +79,6 @@ KirigamiComponents.ConvergentContextMenu {
     Kirigami.Action {
         text: i18nc("@action:inmenu", "Open Secret Backup")
         icon.name: "unlock"
-        visible: NeoChatConfig.secretBackup
         onTriggered: root.window.pageStack.pushDialogLayer(Qt.createComponent('org.kde.neochat', 'UnlockSSSSDialog'), {}, {
             title: i18nc("@title:window", "Open Key Backup")
         })