fix: remove notification title from Dreame Vacuum to avoid duplicated device name

The `notification_title` input defaulted to `{vacuum_name}`, which caused the
device name to appear twice (once in the title, once in the message body that
already starts with `{vacuum_name}`). Drop the title input and revert to
message-only notifications, matching the original behavior.

manifest.json

@@ -1,3 +1,3 @@
 {
-  "version": "2.6.4"
+  "version": "2.9.1"
 }