backup: Mark the backup-destination option as UNUSED

We'll start relying solely on the `backup.lock` file generated by the
`backup-cli`. But we keep the option so we don't automatically break
all configs that simply updated.

backup/backup.py

@@ -335,10 +335,6 @@ def on_init(options: Mapping[str, str], plugin: Plugin, **kwargs):
         apply_write(plugin, c)
 
 
-plugin.add_option(
-    'backup-destination', None,
-    'Destination of the database backups (file:///filename/on/another/disk/).'
-)
 
 
 def kill(message: str):
@@ -366,6 +362,12 @@ def preflight(plugin: Plugin):
     if not os.path.exists("backup.lock"):
         kill("Could not find backup.lock in the lightning-dir")
 
+plugin.add_option(
+    'backup-destination', None,
+    'UNUSED. Kept for backward compatibility only. Please update your configuration to remove this option.'
+)
+
+
 if __name__ == "__main__":
     preflight(plugin)
     # Did we perform the version check of backend versus the first write?