Fix: Remove back navigation from CreateConfirm and CreateBackup (Fixes bitcoin-core#412)

- Removed back button and NavigationBar2 from CreateBackup.qml
- Removed back signal from CreateBackup.qml
- Removed unnecessary imports
- Verified CreateConfirm.qml already had no back navigation
- Maintained forward-only navigation flow with "Done" button

This change ensures users follow a linear wallet creation process
without the ability to navigate backwards, improving the flow
consistency and user experience.

Author: Jaysinh146

Diff for: src/qml/pages/wallet/CreateBackup.qml

@@ -12,21 +12,9 @@ import "../settings"
 
 Page {
     id: root
-    signal back
     signal next
     background: null
 
-    header: NavigationBar2 {
-        id: navbar
-        leftItem: NavButton {
-            iconSource: "image://images/caret-left"
-            text: qsTr("Back")
-            onClicked: {
-                root.back()
-            }
-        }
-    }
-
     ColumnLayout {
         id: columnLayout
         width: Math.min(parent.width, 450)
@@ -89,4 +77,4 @@ Page {
             }
         }
     }
-}
+}