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")
         })

src/devtools/FeatureFlagPage.qml

@@ -21,12 +21,6 @@ FormCard.FormCard {
 
         onToggled: NeoChatConfig.threads = checked
     }
-    FormCard.FormCheckDelegate {
-        text: i18nc("@option:check Enable the matrix 'secret backup' feature", "Secret Backup")
-        checked: NeoChatConfig.secretBackup
-
-        onToggled: NeoChatConfig.secretBackup = checked
-    }
     FormCard.FormCheckDelegate {
         text: i18nc("@option:check Enable the matrix feature to add a phone number as a third party ID", "Add phone numbers as 3PIDs")
         checked: NeoChatConfig.phone3PId