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/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