backups: Don't show enable/disable button as app can't be disabled

Fixes: #2472.

Tests:

- On backups page, the enable/disable toggle button is not visible anymore.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: Veiko Aasa <veiko17@disroot.org>

plinth/modules/backups/__init__.py

@@ -35,6 +35,8 @@ class BackupsApp(app_module.App):
 
     _version = 3
 
+    can_be_disabled = False
+
     def __init__(self) -> None:
         """Create components for the app."""
         super().__init__()